New parser

Reduce string allocations to minimal while parsing markdown and make it
simpler stupid
Add extension points back after refactoring

Extension points were disabled due to major refactoring of the parsing.

Add way to add:
- new block level elements
- new span level elements
cmd: Command Line tool for rendering MD to HTML

User should be able to render markdown text file to html file with same
name

Bug fix:
- reference link keys should be case insensitive
inline: Inline codeblocks

Should parse and render `var code = true;` style codeblocks
nuget: Gist extension nuspec