Add type check to make sure unidecode is being passed a unicode string

[djds23]

@iki I would be happy to work on this and submit a PR.

[iki]

@djds23 do you mean a shortcut that returns original string if it is not instance of unicode on python2, or str on python3?

[djds23]

@iki thanks for asking, I was thinking unicode in python2

[iki]

@djds23 thanks for bringing this on, it's a good idea imo, please submit the PR if you want.

btw, it has to be compatible with both python2/3, so you could probably set the unicode_type on the module level, and then in unidecode function just check if the passed value is instance of it:

import sys
unicode_type = unicode if sys.version_info.major < 3 else str