Fix: Skip emphasis parsing when inside link

[Aaaaash]

Considering we have a link with an underscore like this:

[read_md.md](https://github.com/thetarnav/streaming-markdown)

We might mistakenly parse the _ in the link as an underscore mark, resulting in incorrect insertion of <em> tags in the link.

We would end up with a link formatted like this:

readmd.md](https://github.com/thetarnav/streaming-markdown)

Its DOM structure would resemble:

<a>read<em>md.md](https://github.com/thetarnav/streaming-markdown)）</em></a>

This Pull Request fixes the issue by skipping underscore parsing when encountering _ while the current token is a link.

[thetarnav]

text_with_underline
^
it’s not even specific to links
underscores in words shouldn’t be matched
they probably should require a whitespace before

[mvshmakov]

We are also faced with the same problem, would be cool to see this available in the upstream