[ES] "Una" is not converted to unit

[DaniSalva]

I think this a very useful package but I found a small pitfall in spanish which made me creating an exception in my code. "Una" is not considered and it should be translated to 1, the same case as below:

https://github.com/allo-media/text2num/blob/master/text_to_num/lang/spanish.py
--> Line 50

Actual code:
--> Unit variants
UNITS["un"] = 1

This should be added:
--> Unit variants
UNITS["un"] = 1
UNITS["una"] = 1

Thank you.