Add support for bitbucket.org's markdown, which doesn't allow HTML

Currently docgen creates markdown documentation that contains HTML tags, and it looks weird in [bitbucket](https://bitbucket.org/enikolov/chi/src/624e6e12e9bc3b3073f539dab52abecdec01d769/_examples/rest/routes.md?at=master&fileviewer=file-view-default).

Maybe add an option to disable HTML in the generated output.

Additionally, their syntax for highlighting a line is a bit different from the one at github, e.g. to highlight line 238 of main.go:
github: #L238
bitbucket: #main.go-238