Fix: Error severity for rules with options.

This fixes an issue where rules that accept options would be unable to report an error severity.

As the TODO has and continues to say, we should refactor how we get error severity in the formatters.

Fixes #229
Rule: max-depth

Created the `max-depth` rule, which allows the user to specify the maximum depth that blocks can be nested:

// max-depth: [1, 2]  // Maximum of 2 deep.
function foo() {
  for (;;) {
    if (true) {
      if (true) { // Nested 3 deep.

      }
    }
  }
}

This rule matches the *maxdepth* rule for JSHint.
Merge pull request #233 from iancmyers/fix-severity-for-options

Fix: Error severity for rules with options. (Fixes #229)
Merge pull request #232 from iancmyers/rule-max-depth

Rule: max-depth