Add support for Lists

[JakobDev]

I'm currently using this lib to parse Metadata of debian akcgaes and repo which uses RFC 822. There are some comma separated lists e.g. Tag: devel::rcs, implemented-in::c, interface::commandline, role::program . It is esaiy to convert this into a list, but I think it would be nice if headerparser is able to directly parse a list.

[jwodder]

What do you mean by "able to directly parse a list"? I would expect parsing of lists to be done via parser.add_field(..., type=some_function_for_splitting_on_commas), and the only decent way to simplify that would be if headerparser provided a builtin splitting function for passing to type, but that seems like barely a feature.