Also allow dashes in variables

Looks like some IEEE journals also have a - in their abbreviation names, so this PR allows that.

Before merging, it might be a good idea to break out the this chunk into its own function, but I don't have time to do that tonight.

Also, regarding your question in the other PR, i don't really need a release right now since I can just depend on the git version, and my current bibtex-file doesn't fully parse yet so I'll probably have another PR or two soon :)