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
yes, except `breakpoints` is an array of objects, so you need to loop through it to find where `breakpoint.name == c.min`, and you back to iterating