Fixed headers syntax on Github

In headers, after the hashes (delimiters) a white space is mandatory
before any header character, otherwise it's not recognized as an header
Proper nesting and list item termination in experiment
Headers setext in nesting experiment
Extract content_indentation in nesting experiment
Extract level_indentation in nesting experiment
Rules in nesting experiment
Blockquotes in nesting experiment
Removed useless matchgroup from blockquotes
Single space after list item test case