Output floats for zoom levels in viewport function

[Madelena]

Similar to #5 but to add support for floats for the viewport() function instead of the bounds()function.

Given the new Static Map API supports zoom levels up to two decimal places, it would be great to have an option to yield such value by removing the Math.floor() in the return.

Unlike the bounds() function, this would not require upstream changes in SphericalMercator.

[Scarysize]

I want to open up a PR for this. We probably can't just remove the Math.floor here:
https://github.com/mapbox/geo-viewport/blob/master/index.js#L35-L37

This would lead to a breaking change for everyone who only can work with integer zoom levels. So I guess we hide it behind a flag?

[TeaSeaLancs]

I just opened a PR for this. See #15

Removing the Math.floor does work, i've wrapped it behind a boolean flag. Little unwieldy in that it comes after every other argument, but without major breaking changes I don't see what I could do there.

[MichielDeMey]

@Madelena Fixed using #15