Allow custom function to be specified for splitting/ordering of sub-lists when using --rule-list-split option

[bmish]

Requested here: ota-meshi/eslint-plugin-regexp#493 (comment)

Perhaps we could allow the option to be passed as a function which returns the list of rule lists and their headere.

// .eslint-doc-generatorrc.js
module.exports = {
  ruleListSplit(rules) {
    const sublist1 = rules.filter(...);
    const sublist2 = rules.filter(...);
    return [
      { rules: sublist1, title: 'Rule List 1' },
      { rules: sublist2, title: 'Rule List 2' },
    ];
  }
};

We might want to rename the option from splitBy to split or ruleListSplit for this (or use a separate option).

Possible types we could allow for this option:

Use case	Type	CLI Example	Config File Example
Split by single property	string	--rule-list-split meta.type	ruleListSplit: 'meta.type'
Split by multiple properties (#291)	string[]	--rule-list-split meta.deprecated,meta.docs.category (or pass multiple times?)	ruleListSplit: ['meta.deprecated', 'meta.docs.category']
Custom splitting function	function	(not possible on CLI)	ruleListSplit(rules) { /* see above example */ }

This could be an alternative way to solve #291