Feat: allow Observer.url

[NumberPigeon]

Before this commit:

>>> o
Observer 4 - EnObsB
>>> o.url
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/daydrm/CodePlayground/python/sc2reader/sc2reader/objects.py", line 293, in url
    return self.URL_TEMPLATE.format(
           ^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'region_id'

After this commit:

>>> o = r.observer[4]
>>> o.url
'https://starcraft2.com/en-us/profile/2/1/10400173'