Add support for bold headings

[ismailcherri]

Hi!

We have a Github integration that posts a PR message after doing some checks. The PR message contains a bold heading in markdown:

## Some text, then *some bold text*

This works fine on Github, however, the message on Slack becomes:

*Some text, then *some bold text**

This is due to the lack of support for nested bold tags inside a header tag

I created a fix PR on the html-to-mrkdwn repo, but it seems that this repository is not updated frequently, so I thought this might be the correct place to post the issue

I'm also open to other suggestions related to the matter.

Thanks!