Convert Plugin: Added dont_convert Option

[Stunner]

Addresses #2732. I changed the name from noconvert to a more intuitive dont_convert which I think falls better in-line with the other options of the convert plugin (i.e. never_convert_lossy_files).

[sampsyo]

Nice! This is a really straightforward, simple solution to the problem.

Would you mind adding a quick changelog entry about the feature?

Also, I have to admit that I kind of prefer the shorter name for the option. I’ve never really liked the verbosity of never_etc_etc, which is sort incongruous with the rest of our option names—maybe we should have worked harder on that. And the missing apostrophe in the contraction dont is a little more strange, in my opinion.

Other name options include no_convert and copy_only.

[sampsyo]

This is perfect! ✨ Thanks for implementing this.

[Stunner]

Thank you @sampsyo for being such an awesome and active maintainer of this wonderful project!