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 00330c0 on fernandezcuesta:master
into f0c0c75 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