Added a vim-gutentags extension

Added a segment that displays whether gutentags is running a ctags
instance.

autoload/airline/extensions/gutentags.vim

@@ -0,0 +1,16 @@
+" MIT License. Copyright (c) 2014-2018 Mathias Andersson et al.
+" vim: et ts=2 sts=2 sw=2
+
+scriptencoding utf-8
+
+if !get(g:, 'loaded_gutentags', 0)
+  finish
+endif
+
+function! airline#extensions#gutentags#status()
+  return gutentags#statusline() =~# '^TAGS' ? 'Gen. tags' : ''
+endfunction
+
+function! airline#extensions#gutentags#init(ext)
+  call airline#parts#define_function('gutentags', 'airline#extensions#gutentags#status')
+endfunction