v0.3

This is the third release of nixos-scripts.

Changes:

    * Command: switch
        * is now quiet by default
        * Codebase of the command was refactored
        * updates the nixpkgs now, before generating a tag
        * doesn't tag anything if the command was "build"

    * Command: update-package-def
        * can handle -c and -j for --cores and -j now
        * is now able to push after a successful package update

    * We have a tool for channel updates now
    * show-generation is now able to show generations of a custom profile
    * We have a tool for downloading sources of a package, so the package
      can be installed (from source) without having internet access
    * We have tools for container management now (the 'container' subcommand)
    * A REPL was added
    * Help texts were polished

Some bugfixes and behaviour fixes were included as well.