Fixes stache System plugin.

This fixes the stache System plugin and makes everything work build.
Adds a test that tests EJS, Mustache, and Stache to ensure that
everything builds and adds it to the `grunt test` task. This way in the
future no one will accidentally break Steal compilation. Fixes #1256
Merge pull request #1268 from bitovi/steal-tests

Makes Stache System plugin work when building
Components are now extensible. (#1295)