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)
    $

thefuck/rules/git_tag_force.py

@@ -0,0 +1,13 @@
+from thefuck.utils import replace_argument
+from thefuck.specific.git import git_support
+
+
+@git_support
+def match(command):
+    return ('tag' in command.script_parts
+            and 'already exists' in command.stderr)
+
+
+@git_support
+def get_new_command(command):
+    return replace_argument(command.script, 'tag', 'tag --force')