Add a filter for template directory

[mundschenk-at]

Currently, the directory for Comment Mail templates is hardcoded to <theme>/comment-mail/<filepath>. It would be beneficial if the <theme>/comment-mail/ part could be customized via a filter (e.g. to have cleaner template hierarchies in my themes, I'd like to keep everything under <theme>/templates/).

[raamdev]

@mundschenk-at Thanks for the request. I'll go ahead and mark this as an enhancement that we can add to a future release.