onChange and initialValueSet

[nickevansuk]

This is a great project that we've been recommending to a few others too!

This PR fills a couple of gaps that we needed for our implementation:

• We needed a callback that is called when an item is selected
• We needed the dropdown to respect the initial value of the hidden field when it initialises

This PR implements two new options (which are backwards compatible):

onChange

This is a callback that is called when a selection is made, which includes the value selected.

initialValueSet

This boolean configures the dropdown to use the existing value in the hidden field when it initialises, rather than overwriting it with the default item

[nickevansuk]

Thanks @NeoFusion ! Could you also update https://github.com/NeoFusion/hierarchy-select/tree/gh-pages so we can make use of it in our JSFiddle demos?

[NeoFusion]

@nickevansuk done. Thanks for PR.

[nickevansuk]

Perfect, thanks!