Add PlainTemplate

Raw Html Template
Why do we need this pointless thing, you ask? 
- Because Tilt is the basis for dozens of file-based CMSes and static site generators (such as Hardwired, Nesta, Serve Jekyll, Ruhoh), which rely on tilt to provide all of their supported markup formats

- And... plain HTML is an extremely common thing, especially for people migrating from Wordpress or other content management systems.

Despite it not offering any template functionality (it's not the only one, actually), I think it makes sense to have a null-op html template available, and have a standardized file extension for it.