The Big Refactor

I like that name. :sunglasses:

So, for the source to be fully consistent, these changes are in order:

 - [x] In `html`, the `options` pointer should be named `state`.
 - [x] In `markdown`, the `callbacks` parameter should be named `renderer`.
 - [x] In the `hoedown_markdown` struct, the `cb` member should be `rndr`.
 - [x] In `markdown`, the `rndr_*` utilities should be unprefixed.
 - [x] In `html`, rename `hoedown_html_renderopt` to `rndr_state`.
 - [x] In `markdown`, always use `md` for variable names of `hoedown_markdown`.