Add ack.vim plugin

I have long wanted an easy way to replicate TextMate's "Find in Project",
and this delivers it via a backslash shortcut and selectable results in
a "quickfix" window. If present, Ripgrep or The Silver Searcher (Ag) will
be used instead of Ack for faster search results.

https://github.com/mileszs/ack.vim#keyboard-shortcuts
https://github.com/BurntSushi/ripgrep#whirlwind-tour

bin/bootstrap.sh

@@ -17,7 +17,7 @@ if [ "$OS" = "darwin" ]; then
     [ ! -f /usr/local/bin/brew ] && /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
 
     # Install basic packages
-    for pkg in git grc pixz ssh-copy-id vcprompt wget
+    for pkg in git grc pixz ssh-copy-id vcprompt wget ripgrep
     do
         [ ! -f /usr/local/bin/$pkg ] && brew install $pkg
     done

vim/keys.vim

@@ -1,4 +1,4 @@
-" Directional keys -------------------------------------------------------------
+" Directional keys ------------------------------------------------------------
 
 " When wrapping is on, move cursor by displayed lines instead of file lines
 noremap j gj
@@ -10,3 +10,8 @@ noremap gk k
 noremap H ^
 noremap L $
 vnoremap L g_
+
+" Plugin key bindings ---------------------------------------------------------
+
+" Backslash invokes ack.vim
+nnoremap \ :Ack<SPACE>

vim/plugins.vim

@@ -4,3 +4,11 @@ python from powerline.vim import setup as powerline_setup
 python powerline_setup()
 python del powerline_setup
 set laststatus=2                       " Always show the statusline
+
+" ack.vim: use ripgrep/ag if available ----------------------------------------
+
+if executable('rg')
+    let g:ackprg = 'rg -S --no-heading --vimgrep'
+elseif executable('ag')
+    let g:ackprg = 'ag --vimgrep'
+endif

vim/vundle.vim

@@ -12,6 +12,7 @@ Plugin 'VundleVim/Vundle.vim'
 Plugin 'twerth/ir_black'
 
 " Behaviors
+Plugin 'mileszs/ack.vim'
 Plugin 'matchit.zip'
 Plugin 'tpope/vim-commentary'
 Plugin 'bronson/vim-trailing-whitespace'