Rename the options parameter to settings to avoid confusion with selectize.js's terminology

[dvg-p4]

The documentation for selectize.js refers to the things you can select as "options" and to the parameters that tweak rendering etc. as "settings".

Shiny's createSelectizeInput() and updateSelectizeInput() refer to the things you can select as "choices" and the javascript parameters as "options".

This conflict over what "options" means can be confusing. I suggest renaming the documented parameter to "settings" to match selectize's docs, but leaving options as an alias for backwards-compatibility.