Silence verbose output when VERBOSE environment variable is not set

Hello,

I sometimes run appimages in docker containers, which do not allow mounting of filesystems by default, so I run them with

`APPIMAGE_EXTRACT_AND_RUN=1 NO_CLEANUP=1 `

After the first execution, subsequent runs produce many lines of "File exists and file size matches, skipping". It would be nice if we could silence these informational messages (redirecting `stderr `to `/dev/null` is not ideal as this also redirects error messages from the bundled app).

This string is output [here](https://github.com/AppImage/AppImageKit/blob/master/src/runtime.c#L364), and the function this lives in already has access to a verbosity flag [here](https://github.com/AppImage/AppImageKit/blob/master/src/runtime.c#L277); so it looks reasonably straightforward to implement it. Would this be possible?

Thanks for your work on this!