Add autobpm plugin

This plugin uses librosa to automatically calculate the BPM for a track.
It is based on the keyfinder plugin, and rounds the BPM to an int.

beetsplug/autobpm.py

@@ -0,0 +1,67 @@
+# This file is part of beets.
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+
+"""Uses Librosa to calculate the `bpm` field.
+"""
+
+
+import os.path
+
+from beets import ui
+from beets import util
+from beets.plugins import BeetsPlugin
+
+from librosa import load, beat
+
+class AutoBPMPlugin(BeetsPlugin):
+
+    def __init__(self):
+        super().__init__()
+        self.config.add({
+            'auto': True,
+            'overwrite': False,
+        })
+
+        if self.config['auto'].get(bool):
+            self.import_stages = [self.imported]
+
+    def commands(self):
+        cmd = ui.Subcommand('autobpm',
+                            help='detect and add bpm from audio using Librosa')
+        cmd.func = self.command
+        return [cmd]
+
+    def command(self, lib, opts, args):
+        self.calculate_bpm(lib.items(ui.decargs(args)), write=ui.should_write())
+
+    def imported(self, session, task):
+        self.calculate_bpm(task.imported_items())
+
+    def calculate_bpm(self, items, write=False):
+        overwrite = self.config['overwrite'].get(bool)
+
+        for item in items:
+            if item['bpm'] and not overwrite:
+                continue
+
+            y, sr = load(util.syspath(item.path))
+            tempo, _ = beat.beat_track(y=y, sr=sr)
+            bpm = round(tempo)
+
+            item['bpm'] = bpm
+            self._log.info('added computed bpm {0} for {1}',
+                           bpm, util.displayable_path(item.path))
+
+            if write:
+                item.try_write()
+            item.store()