split ast into a separate crate to write our own implementations which use maud syntax

[vidhanio]

hi there!

i was wondering if you would be interested in a pr separating the maud syntax into its own crate, so authors can write own proc macro which use maud's syntax. this would be a similiar effort to rstml, which just exposes a proc macro html parser and lets you come up with the implementation which transforms it into real code.

[wbcat]

This could help for generating xml ouput #62

[lambda-fairy]

IMO, exposing the parser publicly implies a level of stability that I don't think we have yet. And if you don't care about stability, then you may as well fork the crate anyway.