Added initial class for displaying invisible chars

Summary:
Fixes T11586. First pass at a class for displaying invisible characters. Still need to:

• Write a couple unit tests
• Add some styling to the .invisible-special spans
• Actually start using the class when displaying form errors to users

Currently this makes the string "\nab\x00c\x01d\te\nf" look like:

Test Plan:
Unit tests all pass and run in <1ms:

Reviewers: epriestley, Blessed Reviewers, chad

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley, yelirekim

Maniphest Tasks: T11586

Differential Revision: https://secure.phabricator.com/D16541