include files/fragments ONLY in release mode

[AlexCuse]

I'm thinking about this in the context of jquery-migrate.

It is nice to leave migrateMute set to false on checked in files so you get warnings when debugging, but there isn't a super-easy way to disable it when you go to production (plenty of easy ways, but all the ones I can think of leave some room for error).

So what I'd like to do is be able to append a javascript / css fragment to the very end of the combined file in production mode. Maybe something like Bundle.AppendToCombinedFile(filePath) and Bundle.AppendArbitraryToCombinedFile(content) would make sense? We could also introduce a way to identify assets as "release only".

Some kind of post-processing step might be the best way to get this done, and open the door to being able to support similar needs. I'm just not sure there are enough similar needs to make it worth the effort.