Extensions: indicate, if an extension is external

Show in :AirlineExtensions if the extension comes bundled with airline
or from an external source.

closes #1890

autoload/airline/extensions.vim

@@ -377,6 +377,8 @@ function! airline#extensions#load()
         endif
         try
           call airline#extensions#{name}#init(s:ext)
+          " mark as external
+          call add(s:loaded_ext, name.'*')
         catch
         endtry
       endif

doc/airline.txt

@@ -261,6 +261,7 @@ COMMANDS                                                  *airline-commands*
 
 :AirlineExtensions                                       *:AirlineExtensions*
   Shows the status of all available airline extensions.
+  Extern means, the extensions does not come bundled with Airline.
 
 ==============================================================================
 AUTOCOMMANDS                                          *airline-autocommands*

plugin/airline.vim

@@ -205,11 +205,13 @@ function! s:airline_extensions()
   call map(files, 'fnamemodify(v:val, ":t:r")')
   if !empty(files)
     echohl Title
-    echo printf("%-15s\t%s", "Extension", "Status")
+    echo printf("%-15s\t%s\t%s", "Extension", "Extern", "Status")
     echohl Normal
   endif
   for ext in sort(files)
-    echo printf("%-15s\t%sloaded", ext, (index(loaded, ext) == -1 ? 'not ' : ''))
+    let indx=match(loaded, '^'.ext.'\*\?$')
+    let external = (indx > -1 && loaded[indx] =~ '\*$')
+    echo printf("%-15s\t%s\t%sloaded", ext, external, indx == -1 ? 'not ' : '')
   endfor
 endfunction