Add a key / mapping parameter for extract/extractOne in process

[micostabal]

I don't know if this already exists hereor somewhere else, but it would be cool If the extract methods could have an extra optional, None by default key parameter, just like the sorted function in python.

Imagine being able to extract most similar elements based on attributes or methods of an object for example:

`
class Sth:
a: str
b: int

def __init__(self, _a, _b):
    self.a=_a
    self.b=_b

process.extractOne(Sth('hi', 1), [Sth('bye', 2), Sth('hi!', 3)], key=lambda x: x.a)
`

[maxbachmann]

you should be able to pass this as processor function