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: {F802535}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9408, T7785

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

Author: epriestley

externals/cowsay/ChangeLog

@@ -0,0 +1,31 @@
+3.03	28 May 1999
+	- Added cows/tux.cow, as suggested by xmanoel@i.am
+	- Compatibility with 5.6.0, due to a change in qw().
+	- Renamed devil.cow to daemon.cow, since I know better. :-)
+
+3.02	04 November 1999
+	- Fixed boneheaded code placement so that cowsay -l actually works.
+
+3.01    01 November 1999
+	- Fixed compatibility issues between the Text::Wrap module
+	  that changed between 5.005_02 and 5.005_03.
+	- Fixed tab expansion issues with Text::Tabs.
+
+3.0	13 April 1999, released 14 August 1999
+	- Rewritten into Perl 5 and presented to the world.
+
+-- Not present in CVS from here on down --
+
+2.x	Date?
+	- Arbitrary messages. 
+	- Figlet support (-n). 
+	- Line wrap length (-w). 
+	- Multiple pre-set expressions.
+	- Better arg parsing loop.
+	- Message from stdin or command line.
+
+1.0	Date?
+	- SUBJECT is VERB OBJECT
+
+$Id: ChangeLog,v 1.4 2000/05/29 17:55:24 tony Exp $
+This file is part of cowsay.  (c) 1999-2000 Tony Monroe.

externals/cowsay/INSTALL

@@ -0,0 +1,15 @@
+=================
+Installing cowsay
+=================
+
+If you really want to get things installed a nice and pretty way,
+
+	sh install.sh
+
+It will ask approximately one question.  If you can't answer it,
+you need serious help.
+
+If the install goes well, you can start cowing immediately!  Just
+be sure to read the manual page first...
+
+$Id: INSTALL,v 1.1 1999/08/14 08:03:17 tony Exp $