Add boolean combine for Maybe

[duythinht]

In my opinion we need combine Maybe instance by boolean operator. For example when I want to check a value is None and return Nothing instead of Just(None)

return value and Just(value) or Nothing()

Should be

return Just(value) or Nothing()

[dbrattli]

Thanks, this is great idea! A bit unsure if I want to use .value, (as it is sort of a hack) and perhaps impl. __bool__() for both Nothing and Maybe. But I can change that later if I decide to do it that way.