Replace ag.vim with ack.vim

Based on the comment in the `ag.vim` project:
rking/ag.vim#124 (comment)

.gitmodules

@@ -30,10 +30,6 @@
  path = bundle/syntastic
  url = [email protected]:scrooloose/syntastic.git
  ignore = dirty
-[submodule "bundle/ag"]
- path = bundle/ag
- url = [email protected]:rking/ag.vim.git
- ignore = dirty
 [submodule "bundle/vim-javascript"]
  path = bundle/vim-javascript
  url = [email protected]:pangloss/vim-javascript.git
@@ -68,3 +64,6 @@
 [submodule "bundle/ale"]
  path = bundle/ale
  url = [email protected]:dense-analysis/ale.git
+[submodule "bundle/ack"]
+ path = bundle/ack
+ url = [email protected]:mileszs/ack.vim.git

vimrc

@@ -90,6 +90,12 @@ if executable('ag')
  let g:ctrlp_user_command = 'ag %s -l --nocolor --hidden -g ""'
  " ag is so fast there's no need to cache
  let g:ctrlp_use_caching = 0
+
+ " Enable Ack.vim w/ The Silver Searcher
+ let g:ackprg = 'ag --vimgrep --smart-case'
+
+ " Add alias to rewrite "Ag" to "Ack" to aid in the transition effort.
+ cnoreabbrev Ag Ack
 endif
 
 " vim-fugutive