Service provider for helper global functions

Summary:
We want to allow Blade templates to use custom global functions.

Directives are not appropriated, as they are too heavily cached,
and don't allow mutable state (the value is substed at first run).

Test Plan:

• Run tests
• Tested with an authurl() function in Blade auth templates at D445

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D459