Allow specifying which directive names are processed

[stephenlrandall]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

As seen in #26, sometimes you have text that simply isn't meant to be processed as a directive. This seems mainly to be a problem with textDirective nodes, since -- like other inline Markdown -- there's no requirement for the directive to begin with a non-word character. So you often have things like "I departed at 10:30am", which gets processed unintentionally.

Current solutions

I wrote my own plugin that runs after remark-directive that attempts to restore unintentional textDirective nodes to plain text, although this isn't perfect since it can't exactly reproduce the attributes input.

Proposed solutions

A plugin option that allows a user to specify which directive types should be processed versus left as plain text.

[stephenlrandall]

I am also happy to submit a PR for this if pointed at the right packages to submit the PRs for.