Add mechanism to prevent header ID counter resetting (issue #530)

[Crozzers]

This PR closes #530 by adding a mechanism to prevent the header-ids counter from being reset between conversions. To quote from the issue:

when using markdown2 as part of pdoc [...] two identically-named headers in different docstrings (but the same module/file), [...] will generate an identical anchor

Every time pdoc calls Markdown.convert, the counter that keeps track of header IDs is reset as part of the _setup_extras function

Two proposed solutions were to either stop resetting the ID counter between conversions OR to add an option to disable this behaviour. The latter has been implemented as I think it's less likely to break backwards compatibility.

[nicholasserra]

Thank you!