support custom token "BracketedSpan"

[UnquietCode]

Support a new span token "BracketedSpan". Which looks like: [text]{metadata}

• [x] implement a new token class
• [ ] pass token to our renderer on init
• [ ] define a new render method render_bracketedspan (or maybe render_bracketed_span) which will be invoked by mistletoe
• [ ] for now render it in latex by just rendering the text part, ignoring the metadata
• [ ] test that it renders