Avoid crashing 'beet completion'

Add the `_get_all_options()` method to the argument parser so that `beet
completion` does not throw an error anymore.

Fixes #38

beetsplug/alternatives.py

@@ -13,7 +13,7 @@
 
 import os.path
 import threading
-from argparse import ArgumentParser
+import argparse
 from concurrent import futures
 import six
 
@@ -26,7 +26,6 @@
 
 from beetsplug import convert
 
-
 class AlternativesPlugin(BeetsPlugin):
 
     def __init__(self):
@@ -84,6 +83,16 @@ def parse_args(self, args):
         return self.parser.parse_args(args), []
 
 
+class ArgumentParser(argparse.ArgumentParser):
+    """
+    Facade for ``argparse.ArgumentParser`` so that beets can call
+    `_get_all_options()` to generate shell completion.
+    """
+    def _get_all_options(self):
+        # FIXME return options like ``OptionParser._get_all_options``.
+        return []
+
+
 class External(object):
 
     ADD = 1

test/cli_test.py

@@ -537,3 +537,12 @@ def test_not_removable(self):
             self.assertNotIn('Do you want to create the collection?', out)
         item.load()
         self.assertIn('alt.myexternal', item)
+
+
+class CompletionTest(TestHelper):
+    """Test invocation of ``beet completion`` with this plugin.
+
+    Only ensures that command does not fail.
+    """
+    def test_completion(self):
+        self.runcli('completion')