duplicate resolution prompt (#164)

"Skip" and "keep both" work already, but "remove old" does not. It sets a flag
on the import task object; this flag should then be read by the apply_choices
coroutine, where appropriate action should be taken as part of the same
transaction that adds stuff to the DB.

Unresolved questions:
- Should old files be deleted? (How is this decided? Based on config parameters,
  or with another prompt option?)
- Logging details.
- Folder naming. If I have two albums with the same name, I want them in
  different directories.
- Quiet mode. Currently, there are no checks -- the prompt should not be made in
  quiet mode. (In that case, what should it do?)

beets/importer.py

@@ -308,6 +308,7 @@ class ImportTask(object):
         self.path = path
         self.items = items
         self.sentinel = False
+        self.remove_duplicates = False
 
     @classmethod
     def done_sentinel(cls, toppath):

beets/ui/commands.py

@@ -570,7 +570,21 @@ def resolve_duplicate(task, config):
     """
     log.warn("This %s is already in the library!" %
              ("album" if task.is_album else "item"))
-    task.set_choice(importer.action.SKIP)
+    sel = ui.input_options(
+        ('Skip new', 'Keep both', 'Remove old'),
+        color=config.color
+    )
+    if sel == 's':
+        # Skip new.
+        task.set_choice(importer.action.SKIP)
+    elif sel == 'k':
+        # Keep both. Do nothing; leave the choice intact.
+        pass
+    elif sel == 'r':
+        # Remove old.
+        task.remove_duplictes = True
+    else:
+        assert False
 
 # The import command.