Add separate in-depth documentation for creating custom taglibs

[patrick-steele-idem]

Topics to include:

• Creating nested tags using the following:
  • body-function
  • import-var
• Compile-time transformers
• Compile-time nodes
• Tag renderers
• marko-taglib.json and marko-tag.json

[kristianmandrup]

Please, please please :) Anything we can do to help. Any sample repo to use as reference? Cheers!

[patrick-steele-idem]

The Marko compile-time API is getting a much-needed refactoring with Marko v3. The plan is to finally provide docs for how to extend Marko at compile-time with Marko v3.

[mlrawlings]

Docs for creating custom tags have been added here: http://markojs.com/docs/custom-tags/

We still need docs for compile-time transforms (see #100).