feat: add the ability to ignored keys while sorting

[fernandezcuesta]

Hi, the use case here is to allow nested maps such as:

versions:
  - name: v1alpha1
    capabilities: [ ... ]
    customField: ""
  - name: v1alpha2
    customField: ""

not to be sorted, since there are times when we consider a primary/merge-key useful to remain wherever we define it (usually in 1st place)

[coveralls]

coverage: 99.825%. remained the same
when pulling 1346875 on fernandezcuesta:master
into 8513d9b on adrienverge:master.

[adrienverge]

Hello J.M., thanks for contributing!

The idea looks good. I propose these modifications:

• Name the new option ignored-keys for explicitness (if we add other "ignored" options in the future).
• Accept regexes instead of fixed strings, for more powerful usage. See for example the rule quoted-strings with its options extra-required and extra-allowed.

Also, can you include more tests? Especially for more than 1 ignored-keys, and with the regex feature we should also check it, e.g. name, aaa name and name zzz for ignored-keys: ['name'] vs. ignored-keys: ['^name$'].

Finally, you need to add a .. rubric:: Options in documentation (see other rules for examples).

[adrienverge]

Don't delete this line

[fearphage]

I'm very motivated to get this merged and I see @fernandezcuesta hasn't responded in over a month. Would any be opposed to me making the reviewer comment changes in a new PR? /cc @adrienverge

FYI my use case is GitHub Actions. Sorting some of the keys there makes things more confusing/harder to maintain for humans (read: bad).

[fernandezcuesta]

I'll do it, apologies for the long delay