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

[e-nikolov]

Currently docgen creates markdown documentation that contains HTML tags, and it looks weird in bitbucket.

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

[pkieltyka]

feel free to submit a PR. I don't use bitbucket