Create empty user config file if it does not exist

Running `beet config -e` with a non-existing configuration file
does not always work (e.g., OSX uses `open` by default, which requires
the file to exist). This could occur during e.g. initial setup.

Resolve #1480

beets/ui/commands.py

@@ -1482,11 +1482,13 @@ def config_func(lib, opts, args):
 
 def config_edit():
     """Open a program to edit the user configuration.
+    An empty config file is created if no existing config file exists.
     """
     path = config.user_config_path()
-
     editor = os.environ.get('EDITOR')
     try:
+        if not os.path.isfile(path):
+            open(path, 'w+').close()
         util.interactive_open(path, editor)
     except OSError as exc:
         message = "Could not edit configuration: {0}".format(exc)