Improved markdown parser

[daniel-lerch]

The markdown implementation of v0.9.1 does not hide control chars and is incompatible to CommonMark in terms of bold/italic text.

A full CommonMark parser is not required but *italic*, _italic_, **bold**, __bold__, ***bolditalic*** and ___bolditalic___ should work.

For monospace text escaping like ``$string = "Hello`r`nWorld"`` should render as $string = "Hello`r`nWorld".