continuously compile html template sources

[absurdhero]

This plugin doesn't continuously translate scala.html templates to scala source files.
If it did, then scala:cc could incrementally compile a complete play2 project including templates.

[absurdhero]

It can now monitor for template changes and recompile them. It now needs to invoke scala:cc or Zinc directly to watch for changes in the generated-sources as well as the project source directory.