Added rm -rf support

When someone tries to remove a directory without specifying `-rf`, e.g.

    $ mkdir foo
    $ rm foo

the shell refuses to remove the directory with an error:

    rm: foo: is a directory

Instead, you need to use

    $ rm -rf foo

This rule adds fuck support for this:

    $ mkdir foo
    $ rm foo
    rm: foo: is a directory
    $ fuck
    rm -rf foo

tests/rules/test_rm_dir.py

@@ -0,0 +1,13 @@
+from thefuck.main import Command
+from thefuck.rules.rm_dir import match, get_new_command
+
+
+def test_match():
+    assert match(Command('rm foo', '', 'rm: foo: is a directory'), None)
+    assert not match(Command('rm foo', '', ''), None)
+    assert not match(Command('rm foo', '', 'foo bar baz'), None)
+    assert not match(Command('', '', ''), None)
+
+
+def test_get_new_command():
+    assert get_new_command(Command('rm foo', '', ''), None) == 'rm -rf foo'