Add untracked feature to branch extension.

This will show a little not-existing sign in a buffer,
if that file lives in a git/hg repository but does not exists
there yet. Use `:let g:airline_symbols.notexists='!'` to configure
the '!' as symbol. By default, will use U+2204 symbol

To not impact performance by shelling out a lot, the result is cached
until the buffer is written or a shell command is issued.

Should work with mercurial and git.

fixes #925

doc/airline.txt

@@ -191,6 +191,7 @@ its contents. >
   let g:airline_symbols.paste = 'ρ'
   let g:airline_symbols.paste = 'Þ'
   let g:airline_symbols.paste = '∥'
+  let g:airline_symbols.notexists = '∄'
   let g:airline_symbols.whitespace = 'Ξ'
 
   " powerline symbols