config: Add suport for 'key in config' syntax

We can use this instead of the cumbersome `config[key].exists()`.

beets/util/confit.py

@@ -220,6 +220,9 @@ class ConfigView(object):
         """
         self.set({key: value})
 
+    def __contains__(self, key):
+        return self[key].exists()
+
     def set_args(self, namespace):
         """Overlay parsed command-line arguments, generated by a library
         like argparse or optparse, onto this view's value.