Add Appimage build

[SebastianMpl]

Hi there,

It would be great if you could add an Appimage build for Linux x86_64.
Currently your release requires using sudo to move binaries to certain system directories.
It works fine, although I'm thinking about a way that won't require root.

Thank you,

[mgoltzsche]

Sounds like a great idea btw. I didn't know about AppImage until you mentioned it - very interesting.
I didn't find the time to look into that yet, though, and I won't have the time soon.
Thus, please feel free to create a pull request.
Otherwise I might come back to this but it may take some time.