Add .appimage support and no extension support

[DanHolli]

Repeat of #1
Reason for repeat: issue was closed but not resolved
Why this should be implemented: .appimage files are ready to run across distributions and are a single file installation
How this can be implemented: don't stop install as there's nothing to install. It's an executable file all that needs to be done is copy the file to the game folder and "chmod +x" the file
Difficulty of implementation: Low. Again just copy and mark as executable

Potential issues: version tracking
Potential issue resolution: create a simple file in the game folder denoting the version

Games I've tried playing that won't install due to this issue: OneShot (.appimage) and Super Hexagon (no extension)