Add support for boundless date values

[EvHaus]

In some cases, I want a slider that controls the "end" value or the "start" value, but not both. For example:

• A date range which starts from "any date" (ie. infinity in the past) but up to 2014-01-01, or
• A date range which starts on 2014-01-01 but goes to "any date" (ie. infinity in the past)

The values would look like this:

defaultValues: {min: null, max: new Date(2014, 0, 1)}

defaultValues: {min: new Date(2014, 0, 1), max: null}

In which case the slider would be rendered with only one handle with the other being stuck on the edge of the side.

Bonus points for adding the ability to toggle this behavior between value ranges too. ie. Allow user to select a specific date value or a 'null' value all within the same instance of jQRangeSlider.

[ghusse]

jQRangeSlider does not have this feature, at the moment.