Added functionality for using Ag or Ack

Added a variable that allows the user to specify which operator to
use: grep, ag, or ack.
Updated documentation
Simplified preferred operator logic
Updated documentation
Can now specify array for preferred operator

You can now either specify a string, to use a preferred operator, or a
list. If you specify a list, it loops through it in reverse order and
picks the last one (closest to the front) that is available. No real
point in optimizing and finding the first available and breaking out
of the loop b/c I don't expect many people to provide hundreds of
preferred operators.
Updated documentation for array functionality
Simplified and added comments
Merge pull request #1 from mjwhitta/master

Added support for Ag.vim and Ack.vim