Check if the output format is valid at the beginning

[sezanzeb]

Waiting for this to complete, just to realize that the output format was missing, was a bit frustrating. This checks if a file-extension is valid right at the start.

Other minor changes:

• there was duplicate (and slightly outdated) code in bin/voicefixer
• the shebang was changed to #!/usr/bin/env python
• the description of the modes was added to the argparse help
• Print time it took for the restoration
• Make bin executable

I think it's really great that you provided a docker setup for this.

[sezanzeb]

I still need to test if using an input and output folder still works, but I have to leave now. Other than that, this is finished.

[sezanzeb]

@haoheliu this is ready for review

[haoheliu]

Nice thanks for your effort! I will take a look on that! @sezanzeb