Include "Figlet" and PEAR "Text_Figlet" in externals

Summary:
Ref T7785. This prepares for (but does not yet use) a pure PHP implementation of Figlet parsing and rendering.

Figlet is somewhat complex, but a parser already exists in PEAR. I'll make sure it's suitable and hook it up in the next diff.

Test Plan: N/A, code not reachable

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9408, T7785

Differential Revision: https://secure.phabricator.com/D14101