Add git_tag_force rule

This adds `--force` to `git tag` when needed. For example:

    $ git tag alert
    fatal: tag 'alert' already exists
    $ fuck
    git tag --force alert [enter/↑/↓/ctrl+c]
    Updated tag 'alert' (was dec6956)
    $

tests/rules/test_git_tag_force.py

@@ -0,0 +1,18 @@
+import pytest
+from thefuck.rules.git_tag_force import match, get_new_command
+from tests.utils import Command
+
+
+@pytest.fixture
+def stderr():
+    return '''fatal: tag 'alert' already exists'''
+
+
+def test_match(stderr):
+    assert match(Command('git tag alert', stderr=stderr))
+    assert not match(Command('git tag alert'))
+
+
+def test_get_new_command(stderr):
+    assert get_new_command(Command('git tag alert', stderr=stderr)) \
+           == "git tag --force alert"