Escape hashes less

[hukkin]

Input:

- Recalculate secondary dependencies between rounds (#378)

Current output:

- Recalculate secondary dependencies between rounds (\#378)

Come up with logic that allows us to escape the hash character # in fewer cases. Only escape if the character would start an ATX heading.

[hukkin]

Note: Make a test case to ensure numeric character references (e.g. M) don't break.

[hukkin]

I think the following should do the trick:

1. Move escaping from text renderer to paragraph renderer.
2. Escape first # char of each line if it is only preceded by 0 or more whitespace.