Fixed crash when ignore wasn't specified.

Small bugfix; I didn't specify `ignore` when testing my last patch and it was throwing this error:

```
~/js/uncss/lib/lib.js:100
        if (ignore.indexOf(selector) !== -1) {
                   ^
TypeError: Cannot call method 'indexOf' of undefined
    at ~/js/uncss/lib/lib.js:100:20
    at Array.filter (native)
    at filterUnusedSelectors (~/js/uncss/lib/lib.js:94:22)
    at ~/js/uncss/lib/lib.js:160:17
    at Array.forEach (native)
    at Object.filterUnusedRules (~/js/uncss/lib/lib.js:157:11)
    at uncss (~/js/uncss/lib/uncss.js:81:24)
    at ~/js/uncss/lib/uncss.js:132:13
    at ~/js/uncss/node_modules/async/lib/async.js:229:13
    at ~/js/uncss/node_modules/async/lib/async.js:110:21
```

This makes it so you can ignore the `ignore` option. :-)