Option _dynamicBase for an additional option type: 'function'

I've added another option type to enable the user to define the `_base` variable in even greater detail, as it was possible through the breakpoint map. [Our production media queries](https://github.com/votum/media-queries) are more than a simple `min-width` comparison and I like the idea to have them only in the CSS file.

Now one can compute the `_base` through a user defined function on each execution. I know that this might slow down the script when used without caution (e.g. when using `getComputedStyle()` as in the given example). But on the other hand it was a useful edit to me and might also help others.