add example.vim extension (#134)

autoload/airline/extensions/example.vim

@@ -0,0 +1,47 @@
+" MIT License. Copyright (c) 2013 Bailey Ling.
+" vim: ts=2 sts=2 sw=2 fdm=indent
+
+" Extension specific variables can be defined the usual fashion.
+if !exists('g:airline#extensions#example#number_of_cats')
+  let g:airline#extensions#example#number_of_cats = 42
+endif
+
+" First you should follow the convention and define an 'init' function.
+" It takes a single argument, which is the 'ext'ension manager of sorts,
+" which you can invoke certain functions.  The most important one is
+" 'add_statusline_funcref', which as the name implies, adds a funcref to
+" the collection such that it will be invoked prior to changes being made
+" to the statusline. Finally, invoke this init function in the
+" 'extensions.vim' file after a check to a non-autoloaded variable,
+" command, or function.
+function! airline#extensions#example#init(ext)
+  call a:ext.add_statusline_funcref(function('airline#extensions#example#apply'))
+
+  " Alternatively, you can also modify the default global section by
+  " appending or prepending to it.  But read on to see why using the funcref
+  " method is preferred.
+  let g:airline_section_y .= '%{airline#extensions#example#nyancat()}'
+endfunction
+
+function! airline#extensions#example#apply()
+  " Here we are checking for the filetype, allowing for the extension to
+  " be loaded only in certain cases.
+  if &filetype == "nyancat"
+
+    " Then we define a window-local variable, which overrides the default
+    " g: variable.
+    let w:airline_section_gutter =
+          \ g:airline_section_gutter
+          \ .' %{airline#extensions#example#get_cats()}'
+  endif
+endfunction
+
+" Finally, this function will be invoked from the statusline.
+function! airline#extensions#example#get_cats()
+  let cats = ''
+  for i in range(1, g:airline#extensions#example#number_of_cats)
+    let cats .= ' (,,,)=(^.^)=(,,,) '
+  endfor
+  return cats
+endfunction
+

doc/airline.txt

@@ -267,6 +267,8 @@ For contributions into the plugin, here are the following guidelines:
       branch extension does not follow this convention, but for legacy reasons
       those variables are kept as is.
 
+See the source of |example.vim| for a working extension.
+
 ==============================================================================
 WRITING THEMES                                              *airline-themes*