lint MARK comment formatting

[brennantaylor]

For example enforcing and autocorrecting.

//MARK:bad
// MARK: good

[jpsim]

good idea

[freak4pc]

I wouldn't mind tackling this if I have some spare time tomorrow.

[krodak]

Just added a relevant PR for this

[jpsim]

Done in #768.

[brennantaylor]

THANKS!