Unescape sequences in .cow files which look like escaped sequences

Summary:
Cowsay's .cow files are perl scripts which contain escaped character sequences. This results in an excess of backslashes in the rendered cow. This change replaces occurrences of \x with just x.

This change also introduces conditional unescaping based on whether the cowfile is an original perl-script cowfile or a plain ascii-art file.

Refs T9471

Test Plan:
Use cowsay in a comment and verify that the rendered cow displays appropriately.
Use cowsay with the "dragon" cow and verify that the rendered dragon displays appropriately (contains \@ sequence).

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T9471

Differential Revision: https://secure.phabricator.com/D14170