Initial commit of ruby-mustache

Summary:
Ruby gem to render views and templates.
Required for D6501

Test Plan: Build rcm which depends on ruby-mustache, verify output is as expected.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl, kyrios123

Differential Revision: https://dev.getsol.us/D6646