add `@maybe_threads`

[IanButterworth]

So instead of

if threading
    @threads for x in y
        foo etc.
    end
else
    for x in y
        foo etc.
    end
end

you can do

@maybe_threads threading for x in y
    foo etc.
end

Todo:

• [ ] Handle schedule arg
• [ ] Add tests
• [ ] Add docs

[baggepinnen]

cool :tada: I've wanted something like this for a long time :)