Vimrc additions.

* Setup ,a for ag or ack, depending on what is installed.
  * Prefer ag.
* Center the screen when scrolling search results.
* Adding new shortcuts to edit the plugs file.
Improve the quick search error message.
Turn off the arrow keys.
Turn the arrow keys off in insert mode too.
Configuring plugs.

* Only install ack.vim or ag.vim if the executable is installed.
  * Prefer ag.vim.
* Adding delimitMate.