Use PHP implementation of Cowsay for cowsay rule

Summary:
Ref T7785. Convert the Cowsay Remarkup rule to use a PHP implementation so we don't have to execute an external cowsay binary.

I removed some of the default ".cow" files that come with Cowsay because they:

• include Perl code which we can not interpret; or
• are primarily in-jokes or standalone visual puns or artwork rather than usable actors on the grand stage of cowsay; or
• offended my delicate sensibilities.

Users can add new cows to resources/cows/custom/ if they want to make new cows available.

I have included a majestic original artwork depicting the "Companion Cube" character from Portal.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9408, T7785

Differential Revision: https://secure.phabricator.com/D14100