feat: Pass options to input_selectize

[gshotwell]

This reopens an old pr which had gotten into a rather gnarly git state, and I thought it was easier to just create a new branch.

The comments from the prior PR which were unresolved were:

• Should we come up with an alternative to htmlwidgets::HTML() for passing Javascript options to selectize (Done in Add JS class py-htmltools#81)
• The R code handles a few more things with selectize options, should we copy that behaviour?
• Do we need a test like this one to confirm that plugins can be added.

Fixes #914
Fixes #158

[jcheng5]

Other than the JS class moving from htmltools to shiny, and the other comments I made, LGTM.

[SamEdwardes]

Awesome! This is going to make one of my shiny apps much easier to maintain :)