Add vim-grepper extension

- Displays a segment when a grepper search is running

autoload/airline/extensions/grepper.vim

@@ -0,0 +1,19 @@
+" MIT License. Copyright (c) 2014-2018 Mathias Andersson et al.
+" vim: et ts=2 sts=2 sw=2
+
+" Heavily derived from the Gutentags extension
+
+scriptencoding utf-8
+
+if !get(g:, 'loaded_grepper', 0)
+  finish
+endif
+
+function! airline#extensions#grepper#status()
+  let msg = grepper#statusline()
+  return empty(msg) ? '' : 'grepper'
+endfunction
+
+function! airline#extensions#grepper#init(ext)
+  call airline#parts#define_function('grepper', 'airline#extensions#grepper#status')
+endfunction