Basic work on supporting labels.

Very naïveimplementation of support for labels.  Issue #1

docker_rerun.py

@@ -219,6 +219,15 @@ def modify_image(parser=None, args=None, container=None):
         container.image = args.image
 
 
+def modify_labels(parser=None, args=None, container=None):
+    """Allows labels on the container to be modified."""
+    if parser:
+        parser.add_argument('--label', '-l', action='append',
+                            help='The new label to add to the container.')
+    elif args.label:
+        container.args.extend(['--label %s' % label for label in args.label])
+
+
 def modify_network(parser=None, args=None, container=None):
     """Allows the network to be modified."""
     if parser:

test.py

@@ -81,6 +81,18 @@ def test_modifiers():
            ['docker', 'run', '--name=test123', '--network=host', '-d', 'hello-world'],
            ['--network', 'none'],
            ['docker', 'run', '--name=test123', '--network=none', '-d', 'hello-world'])
+    yield (check,
+           ['docker', 'run', '--name=test123', '-d', 'hello-world'],
+           ['--label', 'testLabel'],
+           ['docker', 'run', '--label=testLabel', '--name=test123', '-d', 'hello-world'])
+    yield (check,
+           ['docker', 'run', '--label=test', '--name=test123', '-d', 'hello-world'],
+           ['--label', 'test2'],
+           ['docker', 'run', '--label=test', '--label=test2', '--name=test123', '-d', 'hello-world'])
+    yield (check,
+           ['docker', 'run', '--label test=1', '--name=test123', '-d', 'hello-world'],
+           ['--label', 'test=2'],
+           ['docker', 'run', '--label test=2', '--name=test123', '-d', 'hello-world'])
 
 
 @with_setup(setup_each, teardown_each)