Add possibility to generate config file containing all defaults and comments

I would like to see an option to generate a “default” config file (maybe with a flag for `beet config`?).
This would help new users with finding out about configs and compiling their own personal configuration, especially if the config options would be annotated directly in the file.

``` yaml
# --- Global Options ---
# These options control beets’ global operation.
# library: Path to the beets library file. By default, beets will use a file
# called library.db alongside your configuration file.
#library: library.db
# directory: The directory to which files will be copied/moved when adding them
# to the library. Defaults to a folder called Music in your home
# directory.
#directory: ~/Music
...
# --- Plugins ---
# See: http://beets.readthedocs.org/en/v1.3.15/plugins/index.html#using-plugins
# plugins: A space-separated list of plugin module names to load.
#plugins: []
...
```

Start by tracing the existing `beet config` command and how default options and plugin settings are represented. Define the generated file's scope and comment format, then add coverage for the option and verify that the output contains the defaults and annotations described in the issue.

Written by the indexing model from the issue text.