Knockdown port (1) - Better semantics

This commit is a part of https://github.com/aziz/knockdown port to MarkdownEditing
Knockdown port (2) - Underscore in words

Makes it possible to write `snake_case_word` without caring about bold or italic. Applies to only GFM.

This commit is a part of https://github.com/aziz/knockdown port to MarkdownEditing
Knockdown port (3) - Give scope to naked URIs

URIs that are not surrounded with `<>`. Applies only to GFM.

This commit is a part of https://github.com/aziz/knockdown port to MarkdownEditing
Update README
Update GFM test
Better highlighting for blockquotes
Improve bold, italic and strikethrough parsing (GFM)

Now they are more similar to GitHub's algorithm.
Add sample markdown
Update README and screenshots