main: Fix flake8 warnings
main: Remove sys.path modification
main: Remove support for --gst-* and --help-gst

Use GStreamer's environment variables instead, e.g. GST_DEBUG=3.
main/settings: Remove --interactive support
Revert "main: Remove sys.path modification"

This reverts commit c00a49ebe1ac1d1e26b7f6ab855eabf27163acde.
Merge pull request #386 from jodal/feature/main-cleanup

Small spring cleanup of __main__