Parametric archive capabilities

I found myself in a situation where I wanted to obtain the actual Archive::Zip Perl object from the call to ->convert(). It also occurred to me that sooner or later we might consider adding capabilities to archive into Tar and other formats (possibly).

So, after some thinking, I came up with a scheme that conservatively changes the current setup - instead of matching 'archive' in the whatsout tests, I match for anything that starts with 'archive'.

Then, we can easily extend the GetArchive logic in LaTeXML::Post::Pack without interfering with any other piece of LaTeXML. For my use case, I have tentatively decided on using the value 'archive::zip::perl' to designate the Perl object of the ZIP archiving workflow. I need access to that object since I need to add a special member to it for the CorTeX processing scheme.

I think this pull request is safe and beneficial to merge. Let me know if you see possible improvements.