Correct and configurable output destination

[zinifier]

It's nice that the default output now is target/release/mybin.AppImage, however:

• cargo-appimage does not respect my custom cargo target-dir (which is shared for all projects)
• i would prefer mybin.AppImage in the crate root like before so maybe adding a output_dir setting can be nice?

I'm happy to make a PR if you think it's good.

PS: The AppDir actually respects the cargo target dir. Just the AppImage doesn't.