Add git_add_force rule

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

    $ git add dist/*.js
    The following paths are ignored by one of your .gitignore files:
    dist/app.js
    dist/background.js
    dist/options.js
    Use -f if you really want to add them.
    $ fuck
    git add --force dist/app.js dist/background.js dist/options.js [enter/↑/↓/ctrl+c]
    $

thefuck/rules/git_add_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 ('add' in command.script_parts
+            and 'Use -f if you really want to add them.' in command.stderr)
+
+
+@git_support
+def get_new_command(command):
+	return replace_argument(command.script, 'add', 'add --force')