Add better documentation

[judofyr]

• [x] Public API for template classes
• [x] Public API for Tilt::Mapping and Tilt
• [ ] Verify that TEMPLATES.md is up to date
• [ ] Document Markdown issues #135

[olleolleolle]

@judofyr Hi, I think that TEMPLATES.md documents Markdown issues quite well in the currently-published version.

https://github.com/rtomayko/tilt/blob/master/docs/TEMPLATES.md#options-4

That checkbox can be ticked.

[olleolleolle]

One checkbox that could be a sub-checkbox to "Verify TEMPLATES.md is up to date" could be "Explain the role of Erubi and ErubiTemplate in the erubis section".

The link is to the commit which added Erubi support. ("With this patch, Erubi will be tried before Erubis and ERB.")