Support for --tags to include/exclude tests based on some optional tagging

:warning:  **Before you read**
*the suggested tagging API has changed since the initial proposal, see new comments further down*

---

As discussed in #928, this is a tentative PR for tagging support in the `bdd` interface. I hope this is not against the guidelines (PR for a new feature), but it will be easier to discuss what's feasible with some visibility on the code changes.

- you can optionally pass an array of tags as the first argument to `describe` and `it`
- by default, all tests always run
- you can specify `--tags` to run a subset of the tests
- tests that don't match the filter are marked as pending

```js
describe('my service', function() {
  it('test 1', function() {});
  describe(['integration'], 'backend connectivity', function() {
    describe(['fast'], 'quick tests', function() {
      it('test 2', function() {});
    });
    it(['slow'], 'test 3', function() {});
  });
});
```

- `mocha` runs everything
- `mocha --tags "not:integration"` runs test `1`
- `mocha --tags "is:integration"` runs tests `2` and `3`
- `mocha --tags "is:integration not:slow"` runs test `2`

You can also programmatically update the filter with 

```js
if (/* out of business hours */) {
  require('mocha').options.tags.add('not:backend');
}
```

If this looks good, happy to look into how to unit test & document it.
Cheers
these two lines can be rewritten as

```js
this.ctx._tags = Array.prototype.concat.apply(this.parent.ctx._tags, arguments);
```