Add detect.isBreakpoint method

Instead of doing something like this

```
/desktop|wide/.test(detect.getBreakpoint());
```

can now do this

```
detect.isBreakpoint({ min: 'desktop' });
```

Another step towards allowing us to add tweakpoints/new breakpoints to our detect object
`initial` will only iterate over the result of `rest`, which mightn't be the whole array

Interested how you would suggest to do this with just `slice`?