Add template subsystem

[noelwelsh]

We need a way to create templates. Twirl is the obvious choice. I think it will mostly require changes in the template project, as Twirl operates as an sbt plugin and therefore must added to the plugins.sbt of the project using it.

Tasks

• [ ] Add and use it to the test project
• [ ] Transfer learning to the template / Krop as needed
• [ ] Document