Namespaced data attributes

To limit the possible collisions between two plugins that make use of similar data attribute names, it is good to namespace our data attributes.

This PR introduces the `select` namespace for all data attributes in the original select element and its options. That is accomplish by reusing data retrival code from eterniccode's [bootstrap-datepicker](https://github.com/eternicode/bootstrap-datepicker/blob/b1775813f4e8e23775b7d389044f9573bc3f4b55/js/bootstrap-datepicker.js#L1263) plugin.

That means that all the data attributes must now be prefixed with `select`, i.e `data-live-search` become `data-select-live-search` (see test.html).

This will allow multiple plugins to use the same option names without issue.

At the moment, the selection is breaking compatibility, so would need to be included into the next major version (according to semver), but backwards compatibility can be easily added to the code.