pass aliases object to check functions.

Simple change that adds flexibility to the check functions.  In my case, I wanted to support a "strict" mode based on the aliases, disallowing anything passed on the command line other than options defined there.  Prevents fat finger abuse :-)

For example:

```
.check(function(argv, aliases) {
  for (var i in Object.keys(argv)) {
    var key = argv[i];
    if (!aliases[key] && key !== '_' && key !== '$0') {
      return false;
    }
  }
  return true;
});
```