Accept unicode strings as input

[Huji]

The replace method in utils.py starts with checking if the input is of type string. This is not ideal because in python 2.x the strings are often of type unicode and that renders some of the functionalities of persiantools unusbale.

For instance, this won't work in persiantools:

digits.fa_to_en(u'۱۲۳۴')

This can be address by allowing the unicode data-type here

[mhajiloo]

It works now 🙏🏻