Expand increment / decrement to non-integer types

[abrassel]

I frequently think that it would be nice if I could increment / decrement non-integer types.

Increment boolean -> toggle true/false.
Increment letter -> lexicographically cycle through a-z.

Thoughts?

[elpixo]

For bools, there's already an attempt at; #2931

Markdown box ticking could be nice too - [x] <-> [ ]