Add `config` command

This will make it easier to get started with beets.

* **`config`** Dumps the user configuration without defaults as YAML.
* **`config -p`** Prints the path of the user configuration.
* **`config -e`** Starts an editor for the user configuration.

If the `--defaults` flag is given the configuration from the beets package is also included

*Since I was to lazy to backport this to master the PR is based on #513.*