[slides.html] Added a feature for building slides with html
[slides.html] Allowed 'slides.html' to be a slides file
[slides.html] Now any file match slides.* can be a slides file

'slides.html' was working all along, but reveal-ck wouldn't go past the
file extension check. That's a bummer.

The new approach is to throw anything that's named 'slides.*' down the
line, and see what happens.

If we also give people a chance to run some ruby.. say.. by loading a
file named `setup.rb` if it is present.. then I suspect that people can
configure new language support without needing any changes in reveal-ck
itself (because Tilt is so flexible).

While we're here, I added error messaging if no file named 'slides.*'
exists. This should fix #11.
[slides.html] Added a feature for building slides with erb

When the approach for "finding the slides files" became "any file that
matches slides.*" we gained supported for slides.html.erb because Tilt
natively knows about erb.
You can now generate slides in vanilla HTML and ERB

Merge branch 'slides.html' into upcoming

* slides.html:
  [slides.html] Added a feature for building slides with erb
  [slides.html] Now any file match slides.* can be a slides file
  [slides.html] Allowed 'slides.html' to be a slides file
  [slides.html] Added a feature for building slides with html