Add token span :map for fenced code blocks

[gaudecker]

Adds :map from the tokenizer to final output. This is useful for referencing specific code blocks from the input.

My use-case for this is for printing pretty errors when evaluating code blocks from within Markdown.

[zampino]

@gaudecker thanks for closing this. We didn't have the need for mapping the input source node-wise so far. I guess you can add your implementation of apply-token directly in your project if needed.

[gaudecker]

Indeed. I'm new to Clojure and didn't think to override the definition locally.