[Proposal] Split output CLI argument into output_dir, format, and label

[SomeoneInParticular]

As the title suggests. Right now, the output flag in the argument parser does not fully match the outputs actually generated by program. It also Doing this split (alongside corresponding documentation) would make it much clearer to the user what files will be produced.

A worked example to demonstrate this:

Current:

> python bibeasy_cli.py -o "testing/test.docx" -t article media
| testing/test-article.docx
| testing/test-media.docx

Proposed

> python bibeasy_cli.py -o "testing/" --format docx -l test -t article media
| testing/test-article.docx
| testing/test-media.docx

This would also allow us to split the "output type checking" code from the output file checking, making everything a bit cleaner and much easier to extend (should Microsoft release yet another Word file format or somesuch)