partial: remove magic-underscoring of partial template names

This logic has proven to be unnecessarily complex.  The partial
should be about rendering with just locals and no layouts - not
the extra wrinkle of expecting underscored template names.

Doing this greatly simplifies the rendering API needed.  Also it
makes template referencing more straightforward and less magical.
You can keep using the underscored-named-partial convention - just
do it manually.
Merge pull request #147 from redding/kr-partial-underscore

partial: remove magic-underscoring of partial template names
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.