Add `--exclude-dir` on dir mode

[KeepWannabe]

I think this feature would be great,

because when i try to bruteforce the directory with logged-in conditions, it will logged-out if i hit the /login route. so for avoid this kind of problem, adding the --exclude-dir/path is awesome.

[firefart]

you can modify the wordlist to exclude the words. Just do a grep -v and pipe it as stdin to gobuster and use -w -