Move formatting code to new module.

librestash/cmd.py

 from argparse import ArgumentParser
-import json
 import logging
 
 from .lib.adaptors.gpx import GpxImporter
 from .lib.exporters.caches import CacheExporter
+from .lib.exporters.formatters import caches_to_json
 from .lib.config import get_default_config_path, get_config
 from .lib.model import get_db
-from .lib.util import json_serialise
 
 logger = logging.getLogger('librestash')
 
@@ -45,4 +44,4 @@ def export_caches():
     data = exporter.export(found_by=args.found_by)
 
     if args.format == 'json':
-        print(json.dumps(data, default=json_serialise))
+        print(caches_to_json(data))

librestash/lib/exporters/formatters.py

+import json
+
+from ..util import json_serialise
+
+def caches_to_json(data):
+    return json.dumps(data, default=json_serialise)