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.

https://github.com/slyshot/maim/commit/8ee2d1afccefbe4d0326d2a548be38abd9e4aa89

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.