Add customization option to disable markdown syntax highlighting

[rrudakov]

Before submitting a PR mark the checkboxes for the items you've done (if you
think a checkbox does not apply, then leave it unchecked):

• The commits are consistent with our contribution guidelines.

• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
• You've updated the readme (if adding/changing user-visible functionality).

Thanks!

---

I would like to introduce a new customization option to disable inline markdown.

After working for some time with clojure-ts-mode on relatively big project I noticed some problems with it. Markdown syntax highlighting is not accurate (note closing backtick has different color in different docstrings):

Also having it enabled sometimes introduces memory leaks. Emacs hangs and consumes all available RAM. Unfortunately I couldn't figure out a reliable recipe to reproduce the memory leak.

[bbatsov]

That's fine by me, but please also open a separate issue about the problems you've encountered, so we won't forget to investigate and fix them eventually.

[rrudakov]

That's fine by me, but please also open a separate issue about the problems you've encountered, so we won't forget to investigate and fix them eventually.

Thanks! I'll create an issue shortly.