experiments with "un-registered" flexattrs
get and set flexattrs with hyphen keys

Here's another little experiment: to make flexattrs a little easier to use for
end users, you can now get and set them by using 'namespace-key' as the
argument to __getattr__ or __setattr__.

For example, try:
$ beet mod foo-bar=baz
$ beet ls -f '${foo-bar}'
baz
baz
baz
...