Allow binding <select>s via data attributes

This attempts to resolve #249 by allowing pre-rendered `<select>` `<option>`s to have values bound from a data attribute (`data-stickit-bind-val`, which was previously used only internally and named `data-stickit_bind_val`), rather than from the `<option value>`.

The tests now verify that pre-rendered `<select>`s without `data-stickit-bind-val` continue to have their options processed to a collection containing string values, and that ones with `data-stickit-bind-val` now support numeric values.

This fixes a longstanding issue with binding pre-rendered selects at the cost of some extra complexity and a bit of duplicative markup.
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