feat: add skip-regex option to cruft update

[sacha-c]

Description

Adds a new skip-regex configuration option which works similar to skip (files) in that it will ignore entire lines of diffs that match the supplied regex.

Comments

I thought of making skip-regex a list of regexes (like skip), but instead kept it as a single regex entry.
I was thinking that this option would probably not be used often so a single entry should be sufficient, and any list functionality can also be done using regex itself.
However if you think a list would be better suited I am happy to make that change

Closes #250

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1a5614f) 99.71% compared to head (f7104e4) 99.72%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   99.71%   99.72%           
=======================================
  Files          21       21           
  Lines        1050     1081   +31     
=======================================
+ Hits         1047     1078   +31     
  Misses          3        3           

Impacted Files	Coverage Δ
cruft/_commands/update.py	98.50% <100.00%> (+0.01%)	⬆️
cruft/_commands/utils/diff.py	100.00% <100.00%> (ø)
cruft/exceptions.py	100.00% <100.00%> (ø)
tests/test_utils.py	100.00% <100.00%> (ø)

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[RobertDeRose]

I would like this feature, but I think a list would be a better option. A few less complicated regexes might be much better than just one complicated one when you need a list.

I would also be nice if the regex could work for files, though, I think the name needs to be different.

I would also say the option name for this feature should be something like skip-diff-regex to make it clear this will skip files based on content, not the files.