Make file extension agnostic and expand the datetime information

[aalexmmaldonado]

Adds the ability to specify supported file types using gitstamp_file_types = ["rst", "md"] in conf.py. (Would close #11.)

Also expands the datetime information to %Y-%m-%d %H:%M:%S %z for additional flexibility with hour and minute.

[jdillard]

Thanks for the PR!

Instead of adding a new gitstamp_file_types config value, I think it would be better to use the native source_suffix. When you use an extension that supports markdown, like myst_parser, it automatically appends ".md", "markdown" to source_suffix, so no additional configuration should be necessary in that case. There is also page_source_suffix available to the template, but I would look into that more to see if it is useful.

I'll have to look over the rest of the PR tomorrow, but I wanted to mention that at first glance.

[aalexmmaldonado]

Ahh, I was not aware of this! That would be a better approach. I will implement it soon.

[jdillard]

Looks good. Thanks!