Feature Request: Support for custom 'when' values

I've been working on a custom plugin with custom rules, and for some of the rules I want to apply to all commit comments, however for other rules I want to only apply for non-WIP commits. I don't believe I can currently do this on a per rule basis.

Expected Behaviour

What I would like to do is something like this (ignore rule names since they refer to custom rules I created):

{
  rules: {
    [commitlintGitHubRules.issueNumberMissing]: [2, 'never-in-non-wips'],
    [commitlintGitHubRules.issueNumberFormat]: [2, 'always'],
    [commitlintGitHubRules.issueNumberDuplicate]: [2, 'never'],

    [commitlintGitHubRules.wipAllowed]: [2, 'always'],

    [commitlintGitHubRules.subjectEmpty]: [2, 'never-in-non-wips'],
    [commitlintGitHubRules.subjectSeparator]: [2, 'always'],
    [commitlintGitHubRules.subjectCase]: [2, 'always-in-non-wips', 'sentence-case'],
    [commitlintGitHubRules.subjectFullStop]: [1, 'always-in-non-wips'],

    [commitlintGitHubRules.typeCase]: [2, 'always', 'lower-case'],
  },
};

Notice how some rules use some new when values to only apply to non-WIP commits, while others apply to all commits.

As you can see, what I would like to do is specify in my configuration always-in-non-wips or never-in-non-wips since my custom plugin identifies whether a commit is a WIP or not and if it is a WIP and the when value is one of these custom values, it just returns [true] as it skips validating the WIP commit.

If the commit isn't a WIP though, then it converts the custom when to the standard equivalent: always-in-non-wips => always; never-in-non-wips => never. It then proceeds to validate accordingly.

In the configuration example above, many of the custom rules delegate to the standard corresponding rules such as subject-case, subject-full-stop, but the custom rules short-circuit for WIP commits if the when value stipulates to only validate non-WIPs. Otherwise it delegates to the standard rules after converting the when to the corresponding standard always and never values as necessary.

Current Behaviour

This all works great with one problem I didn't anticipate - the overall commitlint linter rejects any custom when value in a configuration and so blows everything up, so I can't do this.

Not allowing individual rules to allow for more than two different rule conditions seems quite limiting, and I'm raising this Issue to get some feedback on this.

Suggestion

My suggestion would be to update the linter to avoid globally policing the allowable when values and instead delegate that to individual rules to allow each one to police which values it accepts.

Affected packages

[ ] cli
[x] core
[ ] prompt
[ ] config-angular

Your Environment

Executable	Version
`commitlint --version`	8.3.5
`git --version`	2.25.0
`node --version`	12.16.1

conventional-changelog / commitlint