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: {F802535}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9408, T7785
Differential Revision: https://secure.phabricator.com/D14100
