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

thefuck/rules/rm_dir.py

@@ -0,0 +1,9 @@
+import re
+
+def match(command, settings):
+    return ('rm' in command.script
+            and 'is a directory' in command.stderr)
+
+
+def get_new_command(command, settings):
+    return re.sub('^rm (.*)', 'rm -rf \\1', command.script)