Add a separate `experimentalOptions` table

[Witiko]

Introduction

Since the release of Markdown v2.0.0, a large number of new options was added. However, to retain backwards compatibility, most of these options are disabled by default. This makes it difficult to enable everything back again for a user that is not concerned with backwards compatibility and wants to use the Markdown package with all the bells and whistles.

Proposed solution

Add a separate experimentalOptions table that will have the same structure as the current defaultOptions table, but that will enable the majority of new options and syntax extensions. Add a Lua option experimental that will make the Lua parser use the options in the experimentalOptions table as the defaults.