Add Opt.when method

[halotukozak]

Opt.when is similar to optIf, but takes Opt[?] parameter. The inspiration came from Task.when.

It may simplify code like this:

if (predicate(sth)) sthElse.orElse(sthMoreElse.map(_.sth)) else Opt.Empty

to

Opt.when(predicate(sth)){ sthElse.orElse(sthMoreElse.map(_.sth)) }

[ddworak]

LGTM after nits.

@sebaciv do you want to take a final look?