Feature to exclude security groups by name or regex.

This adds the `-x` flag to exclude security groups - for example
you might have groups that are controlled by Cloudformation that
you don't want Piculet to touch.

You can supply a list of names and/or regular expressions.

With names:

    piculet -x my-sg-one,my-sg-two

To match by regular expression, wrap the name in `'/'`:

    piculet -x "my-sg-one,/^my-excluded-set.*/"