Add support for - filename syntax

[foxpy]

Currently, maim will save image into file named - instead of piping it into stdout, like other Unix utilities do.

This issue superseeds #265.

[slyshot]

Fairly easy to make a patch -- one line in fact.

slyshot@8ee2d1a

Not sure if this is hacky, though. I don't think it corrupts the meaning of "savepathGiven" since - isn't actually meant to be a save path.