Make settings & state buffer local

[alerque]

As of #21 being merged to fix #7 the parser actually loads up and parses only the extensions enabled by user settings.

However ... these settings are only global (g:commonmark#...) and furthermore can only be set once before the Rust module is loaded. Once it is loaded it keeps its own state. Globally.

1. Add buffer local setting options (b:commonmark#...) that can override their global counterparts.

2. Keep the parser state separately for each buffer. Not sure how this will work with Lua's module scoping. We may need to keep a state object either in Lua or keep a hash table of per-buffer options in Rust.

3. Allow updating the settings. If we can't capture the settings being chaged there at least needs to be some way to re-init.