Add musicbrainz id option to importer

* Add '-m', '--musicbrainzid' option to the import command, allowing the user
to specify a single MusicBrainz ID to be used for the candidate lookup instead
of trying to find suitable candidates.
* Modify lookup_candidates() of ImportTask and SingletonImportTask to use the
musicbrainz id if present.

beets/importer.py

@@ -581,8 +581,13 @@ class ImportTask(BaseImportTask):
     def lookup_candidates(self):
         """Retrieve and store candidates for this album.
         """
+        # Use a MusicBrainz id directly if provided by the importer -m option.
+        mb_id = None
+        if config['import']['musicbrainz_id'].exists():
+            mb_id = config['import']['musicbrainz_id'].get()
+
         artist, album, candidates, recommendation = \
-            autotag.tag_album(self.items)
+            autotag.tag_album(self.items, search_id=mb_id)
         self.cur_artist = artist
         self.cur_album = album
         self.candidates = candidates
@@ -821,7 +826,13 @@ class SingletonImportTask(ImportTask):
             plugins.send('item_imported', lib=lib, item=item)
 
     def lookup_candidates(self):
-        candidates, recommendation = autotag.tag_item(self.item)
+        # Use a MusicBrainz id directly if provided by the importer -m option.
+        mb_id = None
+        if config['import']['musicbrainz_id'].exists():
+            mb_id = config['import']['musicbrainz_id'].get()
+
+        candidates, recommendation = autotag.tag_item(self.item,
+                                                      search_id=mb_id)
         self.candidates = candidates
         self.rec = recommendation
 

beets/ui/commands.py

@@ -1022,6 +1022,10 @@ import_cmd.parser.add_option(
     '--pretend', dest='pretend', action='store_true',
     help='just print the files to import'
 )
+import_cmd.parser.add_option(
+    '-m', '--musicbrainzid', dest='musicbrainz_id',
+    help='restrict the matching to a single MusicBrainz id'
+)
 import_cmd.func = import_func
 default_commands.append(import_cmd)