Add interface fallback (Powerline>Unicode>ASCII)

Basically what the title says. First check if the user has Powerline,
fall back to Unicode symbols if he doesn't and fall back to ASCII
symbols if he doesn't have that either.

autoload/airline/init.vim

@@ -17,10 +17,10 @@ function! airline#init#bootstrap()
   let s:loaded = 1
 
   let g:airline#init#bootstrapping = 1
-  call s:check_defined('g:airline_left_sep', get(g:, 'airline_powerline_fonts', 0) ? "\ue0b0" : " ")
+
-  call s:check_defined('g:airline_left_alt_sep', get(g:, 'airline_powerline_fonts', 0) ? "\ue0b1" : "|")
+  let g:airline#util#async = v:version >= 800 && has('job')
-  call s:check_defined('g:airline_right_sep', get(g:, 'airline_powerline_fonts', 0) ? "\ue0b2" : " ")
+  let g:airline#util#is_windows = has('win32') || has('win64')
-  call s:check_defined('g:airline_right_alt_sep', get(g:, 'airline_powerline_fonts', 0) ? "\ue0b3" : "|")
+
   call s:check_defined('g:airline_detect_modified', 1)
   call s:check_defined('g:airline_detect_paste', 1)
   call s:check_defined('g:airline_detect_crypt', 1)
@@ -32,9 +32,6 @@ function! airline#init#bootstrap()
   call s:check_defined('g:airline_exclude_preview', 0)
   call s:check_defined('g:airline_gui_mode', airline#init#gui_mode())
 
-  let g:airline#util#async = v:version >= 800 && has('job')
-  let g:airline#util#is_windows = has('win32') || has('win64')
-
   call s:check_defined('g:airline_mode_map', {})
   call extend(g:airline_mode_map, {
         \ '__' : '------',
@@ -64,20 +61,65 @@ function! airline#init#bootstrap()
         \ }, 'keep')
 
   call s:check_defined('g:airline_symbols', {})
+  " First define the symbols,
+  " that are common in Powerline/Unicode/ASCII mode,
+  " then add specific symbols for either mode
   call extend(g:airline_symbols, {
           \ 'paste': 'PASTE',
           \ 'spell': 'SPELL',
-        \ 'readonly': get(g:, 'airline_powerline_fonts', 0) ? "\ue0a2" : 'RO',
-        \ 'whitespace': get(g:, 'airline_powerline_fonts', 0) ? "\u2632" : "\u2632",
-        \ 'linenr': get(g:, 'airline_powerline_fonts', 0) ? "\ue0a1" : "\u33D1",
-        \ 'maxlinenr': get(g:, 'airline_powerline_fonts', 0) ? "\u2630" : "\u2630",
-        \ 'branch': get(g:, 'airline_powerline_fonts', 0) ? "\ue0a0" : "\u16A0",
-        \ 'notexists': "\u2204",
           \ 'modified': '+',
-        \ 'space': ' ',
+          \ 'space': ' '
-        \ 'crypt': get(g:, 'airline_crypt_symbol', nr2char(0x1F512)),
           \  }, 'keep')
 
+  if exists('g:airline_powerline_fonts')
+    " Symbols for Powerline terminals
+    call s:check_defined('g:airline_left_sep', "\ue0b0")      " 
+    call s:check_defined('g:airline_left_alt_sep', "\ue0b1")  " 
+    call s:check_defined('g:airline_right_sep', "\ue0b2")     " 
+    call s:check_defined('g:airline_right_alt_sep', "\ue0b3") " 
+    " ro=, ws=☲, lnr=☰, mlnr=, br=, nx=Ɇ, crypt=🔒
+    call extend(g:airline_symbols, {
+          \ 'readonly': "\ue0a2",
+          \ 'whitespace': "\u2632",
+          \ 'linenr': "\u2630",
+          \ 'maxlinenr': "\ue0a1",
+          \ 'branch': "\ue0a0",
+          \ 'notexists': "\u0246",
+          \ 'crypt': nr2char(0x1F512),
+          \ }, 'keep')
+  elseif &encoding==?'utf-8'
+    " Symbols for Unicode terminals
+    call s:check_defined('g:airline_left_sep', "")
+    call s:check_defined('g:airline_left_alt_sep', "")
+    call s:check_defined('g:airline_right_sep', "")
+    call s:check_defined('g:airline_right_alt_sep', "")
+    " ro=⊝, ws=☲, lnr=☰, mlnr=㏑, br=ᚠ, nx=Ɇ, crypt=🔒
+    call extend(g:airline_symbols, {
+          \ 'readonly': "\u229D",
+          \ 'whitespace': "\u2632",
+          \ 'linenr': "\u2630",
+          \ 'maxlinenr': "\u33D1",
+          \ 'branch': "\u16A0",
+          \ 'notexists': "\u0246",
+          \ 'crypt': nr2char(0x1F512),
+          \ }, 'keep')
+  else
+    " Symbols for ASCII terminals
+    call s:check_defined('g:airline_left_sep', "")
+    call s:check_defined('g:airline_left_alt_sep', "")
+    call s:check_defined('g:airline_right_sep', "")
+    call s:check_defined('g:airline_right_alt_sep', "")
+    call extend(g:airline_symbols, {
+          \ 'readonly': 'RO',
+          \ 'whitespace': '!',
+          \ 'linenr': 'ln',
+          \ 'maxlinenr': ':',
+          \ 'branch': '',
+          \ 'notexists': '?',
+          \ 'crypt': 'cr',
+          \ }, 'keep')
+  endif
+
   call airline#parts#define('mode', {
         \ 'function': 'airline#parts#mode',
         \ 'accent': 'bold',