Add snag identifier to SIMPLEXpress

Summary:
Add snag bool to UnitAttributes, add test case

Update formatting in Unit constructor and tests

Add snag character to ReservedCharacter, clarify comments

Logic in UnitParser::parse() for both snag and standard unit

Test Plan: Included Goldilocks tests

Reviewers: jcmcdonald, wdede

Reviewed By: jcmcdonald

Subscribers: wdede, jcmcdonald, ardunster

Tags: Programming [Dept], SIMPLEXpress [Project]

Maniphest Tasks: T1408

Revert Plan: na

Differential Revision: https://phabricator.mousepawmedia.net/D363