Add support for detecting aliases of VLC, MPV etc.

I'm using Bazzite, and the usual way to install programs is to use Flatpaks (or AppImages). The standard way to expose Flatpaks on the command line is to use bash aliases.

I've added an alias for vlc in my bash profile, so I can invoke vlc from the command line. However, yewtube does not recognize it because it is searching for an executable file in the PATH in this function:

https://github.com/mps-youtube/yewtube/blob/master/mps_youtube/util.py#L65

It would be nice if the alias could be detected.