allow custom key assignment to variables

[fybx]

To better fit any project's requirements, any variable that is passed using keys should support

• assignable using only a short-form key
• assignable using only a long-form key
• assignable using both
• assignable using non-matching long or short form key

Example cases:

1. c.add_variable(name=age, type=int, shortform='-b', longform=None)
   Accepts:

• -b value
• -b=value

2. c.add_variable(name=path, type=str, shortform='p', longform='pathTo')
   Accepts:

• -p value or -p=value
• --pathTo value or --pathTo=value