New extras file for Web properties

[fwkoch]

For now, this only includes properties.extras.URL - I suspect properties.extras.Email would also be useful.

[codecov]

Codecov Report

Merging #257 into dev will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #257      +/-   ##
==========================================
+ Coverage   96.24%   96.28%   +0.04%     
==========================================
  Files          16       17       +1     
  Lines        2318     2345      +27     
==========================================
+ Hits         2231     2258      +27     
  Misses         87       87

Impacted Files	Coverage Δ
properties/extras/__init__.py	100% <100%> (ø)	⬆️
properties/extras/web.py	100% <100%> (ø)

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