Total revamp of the auto-format feature

[fredck]

The auto-format feature has been re-written from scratch with the following changes:

• It's more strict when matching inline formatting markers:
  • The opening and closing markers must be touching a word. E.g. **bold** vs ** non-bold **.
  • No cross styling matches. E.g. [bo_ld] outside bold_ (no match), _before [bold] after_ (match).
  • No match inside inline code or code-blocks.
• It allows for the formatting of paragraphs with contents. E.g. type *  at the beginning of a paragraph.
• It has a better API:
  • Allows for easily registering new auto-formatters.
  • Allows for extending it with new types of auto-formatters (other than the built-in "inline" and "block").
• Performance optimizations.

Fixes #34.