Custom heading ids

Sometimes it may be useful to be able to assign custom heading ids to .md headings. For example, if heading name is too long or contains special characters or math formulas, it may become tedious to reference. In this case, a heading may be given a custom id like this:

## My header with math formulas \` E = mc^2 \` {#my-custom-heading-id}

The support from custom heading ids in this documentation is enabled by remark-custom-heading-id