Add field exclusion list to drop-all-caps

[mcsitter]

If drop all caps option is checked, a field is made available to define fields for exclusion. The CLI flag also accepts a list of fields to exclude.

If it is okay the way it is i am happy to expand the drop-all-caps test case.

Closes #24 Closes #49

[FlamingTempura]

Thanks for the contribution!

Seems like a good idea. However, I suggest the option should accept an include list instead of an exclude list so that it's consistent with other options. We can provide a reasonable list of defaults: title, journal, booktitle, etc.

Unless there's a reason that an exclude list makes more sense?