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
Something along the lines of
`breakpoints.slice(breakpoints.indexOf(c.min), breakpoints.indexOf(c.max)).contains(currentBreakpoint)` (4 native iterations) or
`var i = breakpoints.indexOf(currentBreakpoint);
i >= breakpoints.indexOf(c.min) && i <= breakpoints.indexOf(c.max)` (3 native iterations, less garbage)