Retina icons

My fiancée, [Jessica Zehring][jz], and I made retina ready icons for Total Terminal. We included the original XCF (GIMP) files, the `VisorActive@2x.png` and `VisorInactive@2x.png`, as well as highlight versions of both.

This pull resolves issue #65.

[jz]: https://www.vizify.com/jessica-zehring