Add vim-grepper extension

- Displays a segment when a grepper search is running

doc/airline.txt

@@ -1002,6 +1002,12 @@ vim-gutentags <https://github.com/ludovicchabant/vim-gutentags>
 * enable/disable vim-gutentags integration >
   let g:airline#extensions#gutentags#enabled = 1
 
+-------------------------------------                     *airline-grepper*
+vim-grepper <https://github.com/mhinz/vim-grepper>
+
+* enable/disable vim-grepper integration >
+  let g:airline#extensions#grepper#enabled = 1
+
 -------------------------------------                     *airline-capslock*
 vim-capslock <https://github.com/tpope/vim-capslock>