Feature: magic help comment

[ghost]

As @jwoertink said in #119, since "magic comments" are now used for control instead of the old commands, a magic help comment could be cool, that would document all the available magic comments. Operation could be something like:

#exit, #quit - Exit ICR
#paste - Enter paste mode

Or similar. I will probably try and PR this later this week, but if anyone else wants to try it instead that's fine too.

[ghost]

Should be fixed by #123.