documentation expansion for config command (#552)
backport YAML dumping from Confit (#552)

We actually added more full-blown YAML dumping to the Confit library a while
back but it looks like it never made it into beets. It offers a few benefits
over the hand-rolled flattening that the `config` command was previously
using, including printing ordered dicts in the right order. But it also
appears to have broken logic when attempting to hide defaults. I'll fix this
right quick.
config.dump: no filename parameter

It's bad style, in my opinion, to provide a file-writing option when the
client code could just as easily write the string to the file. (It implies
we're doing some sort of streaming, which we're obviously not.)
config.dump: new approach to excluding defaults

This uses a lot less code (and actually works).