optional auto_identifiers output when outputting commonmark_x

[kylecordes]

Describe your proposed improvement and the problem it solves.

People use pandoc in myriad ways. Example: I keep the official version of some documentation in markdown (ideally something close to commonmark), but occasionally round-trip it (using pandoc) back and forth to DOCX. (Then to Google Docs and back, for online collaborative editing. Long story.)

This mostly works great, but when it comes back (DOCX->MD), the markdown files are full of superfluous auto_identifiers. For example, I want:

### Class setup

but I get:

### Class setup {#class-setup}

This appears, based on various issue comments, to be because the auto_identifier feature is default enabled with commonmark_x.

Therefore a feature request:

Separate emitting automatic identifiers when translating from commonmark_x to something else, from writing the identifiers to commonmark_x output.

Describe alternatives you've considered.

I considered keeping the extra identifiers in my canonical-source-of-docs markdown, and decided it is too distracting.

I considered a sed+refex to strip the noise out, and did that.

[jgm]

I can fix this.