Email subject configuration options

[mcwumbly (Dave McClure)]

For reference, the configurable subject line was discussed here:

Show categories in e-mail notifications

@eviltrout can you implement a configurable substitution scheme string for email titles this week including {category}, {sitename}, {topictitle}, etc? Just the title of notification emails should be configurable in this manner.

And some of the other details regarding threading in mail clients like outlook being discussed here:

Emails are not threaded in Outlook 2013

When turning on emails for every post to a forum, the emails do not thread correctly, at least in Outlook 2013. It looks like there’s an attempt to make it work, based on the In-Reply-To headers, but somehow it’s still not working :(. I imagine Outlook has some strange heuristics you might have to appease…