Perform template field substitution on albums

- adds another traversal through all plugins' template_fields for each
  'evaluate_template' call.
- requires the following idiom (or equivalent):

    @Plugin.template_field(field')
    def _tmpl_field(album):
        """Return stuff.
        """
        if isinstance(album, Album):
            return stuff
Append plugin template fields to 'beet fields' output with -p, --plugins
Use newly defined '$missing' template field instead of hardcoded format (-c)
require py-mb-ngs 0.4

The MusicBrainz search API changed. This version fixes compatibility with
that, so changing our dependency version will make users upgrade automatically
when upgrading to 1.1.1.
Initial import of 'duplicates' plugin
Merge pull request #274 from pedros/album_template_fields

Perform template field substitution on albums
changelog/API docs for #274
Merge pull request #275 from pedros/show_plugin_fields

Append plugin template fields to 'beet fields' output with -p, --plugins
changelog and tweaks for #275

I've removed the -p option. The command now always shows plugin-provided
template fields if any are available. We also avoid printing out blank lines
for plugins that don't provide fields.
Merge pull request #276 from pedros/missing

Use newly defined '$missing' template field instead of hardcoded format (-c)
doc correction and refactoring for #275

Plugin-provided template fields are not available for queries. (Perhaps they
should be, though!)
fix inline for album plugin fields (#274)
Merge pull request #278 from pedros/duplicates

Duplicates plugin
merge
thanks and doc tweaks for duplicates (#278)
duplicates: refactoring and empty-ID fix (#278)

The main change here, aside from documentation/naming updates, is that we skip
"duplicates" that arise from albums/tracks that are missing their MBIDs.