Merge pull request #136 from redding/kr-partial-tweak

don't add an underscore to a partial's basename if has one already
set render template opts so that they can be easily overridden

The previous implmentation made it more difficult to override with
custom opt values.  Specifically, the `:layout` opts was set using
`||=` so if you wanted to manually set it to `false` you couldn't.

This is all prep for reworking how rendering works and moving away
from Sinatra.