Support for it.only() and xit(), and the describe equivalents

@TabDigital/api (as taken from the new [README](https://github.com/TabDigital/mocha-filter/blob/only-and-skip/README.md)).

To help with quick debugging, filtering is disabled on any test that uses
`it.only(...)` or `describe.only(...)`.

```js
filter.tag('slow').it.only('does something', function() { /* ... */ });
// runs even if MOCHA_FILTER="exclude(slow)"
```

Similarly, if you ignore a test with `xit` or `xdescribe`, it will be ignored
regardless of the runtime configuration.

```js
filter.tag('fast').xit('does something', function() { /* ... */ });
// ignored even if MOCHA_FILTER="include(fast)"
```