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]
    $

tests/rules/test_git_add_force.py

@@ -0,0 +1,22 @@
+import pytest
+from thefuck.rules.git_add_force import match, get_new_command
+from tests.utils import Command
+
+
+@pytest.fixture
+def stderr():
+    return ('The following paths are ignored by one of your .gitignore files:\n'
+            'dist/app.js\n'
+            'dist/background.js\n'
+            'dist/options.js\n'
+            'Use -f if you really want to add them.\n')
+
+
+def test_match(stderr):
+    assert match(Command('git add dist/*.js', stderr=stderr))
+    assert not match(Command('git add dist/*.js'))
+
+
+def test_get_new_command(stderr):
+    assert get_new_command(Command('git add dist/*.js', stderr=stderr)) \
+           == "git add --force dist/*.js"