Handle weird characters in the field's name

[Former user]

(alternatively called tag, probably).

I couldn't find a proper spec for the bibtex format, so this might be moot.

Recently the web of science website1 started putting extra fields that have '(' or ' ' in the name:

Usage-Count-(Last-180-days) = {{4}},

And that breaks pybtex. In my app, I read the file, replace those and parse the resulting string, but that's a kludge, a bad one at that.

So, if those things are supported by the spec, pybtex should be able to load them(something like r'$[^\=]*' ), if not at least I have a spec to reference when I raise this with web of science.