Add ag mappings in vim

https://robots.thoughtbot.com/faster-grepping-in-vim

- Create an `:Ag` command if none is defined.
- map `\` in normal mode to set up the command for an argument

vimrc

@@ -72,6 +72,11 @@ if executable('ag')
 
   " ag is fast enough that CtrlP doesn't need to cache
   let g:ctrlp_use_caching = 0
+
+  if !exists(":Ag")
+    command -nargs=+ -complete=file -bar Ag silent! grep! <args>|cwindow|redraw!
+    nnoremap \ :Ag<SPACE>
+  endif
 endif
 
 " Make it obvious where 80 characters is