branch: display dirty state

If the repository is considered dirty, do display the
g:airline_symbols.dirty symbol right after the branch name.

autoload/airline/init.vim

@@ -86,7 +86,7 @@ function! airline#init#bootstrap()
     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=🔒
+    " ro=, ws=☲, lnr=☰, mlnr=, br=, nx=Ɇ, crypt=🔒, dirty=⚡
     call extend(g:airline_symbols, {
           \ 'readonly': "\ue0a2",
           \ 'whitespace': "\u2632",
@@ -94,6 +94,7 @@ function! airline#init#bootstrap()
           \ 'maxlinenr': " \ue0a1",
           \ 'branch': "\ue0a0",
           \ 'notexists': "\u0246",
+          \ 'dirty': "\u26a1",
           \ 'crypt': nr2char(0x1F512),
           \ }, 'keep')
   elseif &encoding==?'utf-8' && !get(g:, "airline_symbols_ascii", 0)
@@ -111,6 +112,7 @@ function! airline#init#bootstrap()
           \ 'branch': "\u16A0",
           \ 'notexists': "\u0246",
           \ 'crypt': nr2char(0x1F512),
+          \ 'dirty': '!',
           \ }, 'keep')
   else
     " Symbols for ASCII terminals