Add a magic comment syntax to set MIRIFLAGS

[saethlin]

Based on https://github.com/integer32llc/rust-playground/issues/446#issuecomment-670917927

I really want to be able to set MIRIFLAGS when running code in the playground. The community Discord uses a bot that accesses the playground and we use it for demos. It's a real bummer that any time -Zmiri-tag-raw-pointers is required to demonstrate something, we can't use the bot. I don't think changing the defaults is a particularly good option either, because we may want to make a point about the impact of -Zmiri-tag-raw-pointers. And I recently added -Zmiri-backtrace which should be left as its default, but in a pinch can remove backtraces entirely to make the output fit in Discord message length limit.

[thomcc]

Related, but probably is more difficult to handle (and can't be bolted on here): https://github.com/integer32llc/rust-playground/issues/698

[Noratrieb]

Any updates on this?