vim: use ack.vim instead of ag.vim

ag.vim is deprecated (rking/ag.vim#124). Use ack.vim with the proper
glue to use it with ag.

vimrc

@@ -165,7 +165,7 @@ if isdirectory($HOME . "/.vim/bundle/Vundle.vim")
 
     Plugin 'chrisbra/SudoEdit.vim'
     Plugin 'ervandew/supertab'
-    Plugin 'rking/ag.vim'
+    Plugin 'mileszs/ack.vim'
     Plugin 'scrooloose/nerdcommenter'
     " Until https://github.com/tpope/vim-fugitive/pull/1381 is merged.
     Plugin 'osandov/vim-fugitive'
@@ -193,6 +193,20 @@ endif
 " SuperTab scroll down
 let g:SuperTabDefaultCompletionType = "<C-N>"
 
+" Make ack.vim use ag.
+let g:ackprg = 'ag --vimgrep'
+" Add Ag aliases for all of the Ack commands.
+command! -bang -nargs=* -complete=file Ag           call ack#Ack('grep<bang>', <q-args>)
+command! -bang -nargs=* -complete=file AgAdd        call ack#Ack('grepadd<bang>', <q-args>)
+command! -bang -nargs=* -complete=file AgFromSearch call ack#AckFromSearch('grep<bang>', <q-args>)
+command! -bang -nargs=* -complete=file LAg          call ack#Ack('lgrep<bang>', <q-args>)
+command! -bang -nargs=* -complete=file LAgAdd       call ack#Ack('lgrepadd<bang>', <q-args>)
+command! -bang -nargs=* -complete=file AgFile       call ack#Ack('grep<bang> -g', <q-args>)
+command! -bang -nargs=* -complete=help AgHelp       call ack#AckHelp('grep<bang>', <q-args>)
+command! -bang -nargs=* -complete=help LAgHelp      call ack#AckHelp('lgrep<bang>', <q-args>)
+command! -bang -nargs=*                AgWindow     call ack#AckWindow('grep<bang>', <q-args>)
+command! -bang -nargs=*                LAgWindow    call ack#AckWindow('lgrep<bang>', <q-args>)
+
 " Plugin autocmds
 augroup vimrcPlugin
     au!