add support for rectanular radius

[alexrashed]

Currently it is not easily possible to create a rectangular shape from coordinates.
This PR simply extends the get_boundary function to also accept a tuple for the radius (to define "x-radius" and "y-radius").
This is a bit of a hacky solution, but I hacked it into my fork for a small project, and I thought it might make sense to file a PR.
Feel free to drop / close this PR if it's too hacky ;)

Here's a test-snippet for the barcelona example from the README:

from prettymaps import *
from matplotlib import pyplot as plt

plot(
    (41.39491,2.17557),
    radius=(500, 707),
    figsize=(8.27, 11.69),
    preset="barcelona",
    save_as=f'barcelona.svg'
)

This would result in the following image:

Fixes #81

PS.: Thanks for this awesome OSS project! 😍