refactor media module, change --env to --root

This commit moves the media scanner out of the db module and into
a new module, media. We also change the --env parameter to --root,
which takes a path to a directory where the default configuration
and the transcoded media cache will live.

test/test_scanner.py

@@ -5,7 +5,8 @@ from unittest.mock import MagicMock
 from sqlalchemy import func
 
 import groove.exceptions
-from groove.db import scanner, track
+from groove.media import scanner
+from groove.db import track
 
 
 def test_scanner(monkeypatch, in_memory_db):