sinatra runner: render with template source if there is an engine

This updates the sinatra runner to prefer to render using a matching
engine on the template source if one has been configured.  If not
it falls back to rendering with Sinatra as before.

This is a temporary patch to allow transitioning off sinatra and
on to engines.  This is part of moving away from Sinatra for rendering.

@jcredding ready for review.  I chose not to add a system test for this and the tests will be dramatically changing once I remove all the sinatra rendering logic out of here.  I'll address any necessary system tests for template source rendering then.