Rule: max-depth

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