Support auto-generated config lists

[bmish]

Fixes #61.

This is an optional feature for anyone who adds these marker comments to their README.md:

<!-- begin auto-generated configs list -->
<!-- end auto-generated configs list -->

Rendered example of the table/list that will be generated:

	Name	Description
✅	recommended	These rules are recommended for everyone.
🎨	stylistic	These rules are more about code style than bugs.
⌨️	typescript	These are good rules to use with TypeScript.

Config descriptions will be included in the list for configs that export a description property. This is not yet an official ESLint config property but it should likely become one.

Future follow-up features to consider:

• Configurable columns with an option --config-list-columns if we add more columns Consider addition config list columns #490
• Columns for count of rules enabled, set to warn, or disabled in each config Consider addition config list columns #490
• Automatically add this list in ## Configs section even when marker comments aren't present yet (breaking change) Consider unified option for controlling what features are required #488
• feat: add auto-generated configs list to readme in template eslint/generator-eslint#164

This feature respects the following options: --config-format, --ignore-config, --path-rule-list (may need to be renamed #489). The URL from --url-configs is not currently used in this feature.