Support a user-defined function for --rule-list-split option

[bmish]

Fixes #294.

The implementation of this is pretty simple because we now allow the user to provide the same function we use internally to split lists. The more complex part is dealing with the various kinds of values that are now allowed for this option.

TODO:

• Add more documentation
• Add more comments to new types
• Too many disable comments in validation code
• Throw if function returns no sections or no rules in a section
• Throw if a list contains the same rule twice
• Use ajv to validate return value of user-defined function

Open question:

• Should the function return value include the list of tuples [ruleName, rule] which is the same format it is passed, or should it just include the list of rule name strings? I have chosen to stick with the tuples because this allows very simple filtering without having to map the filtering results to the rule name.

Potential follow-up

• Improve README organization
• Fix TODO comment regarding workaround to avoid losing function during merge of options in cli.ts
• Figure out how to test the JavaScript-based config file .eslint-doc-generatorrc.js #366