nicer handling of matched brackets in link labels

[yshavit]

This is a followup to #88. For the resolution on that one, we just escape all brackets, in both img alt and link labels. This is correct, but a nicer formatting would be to find matched brackets and not escape them.

The following two links are equivalent:

[link [foo [bar]]](https://example.com)
[link \[foo \[bar\]\]](https://example.com)

Today, we always output the bottom one; but where we can, it'd be nice to output the top one instead.