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 #232 from iancmyers/rule-max-depth

Rule: max-depth