server-side selectize input

There are two issues in this PR:

1. Does anyone feel uncomfortable with `session$.impl` in `updateSelectizeInput()`?
2. The selectize.js library does not support regular expressions or case-sensitive searching, but it is fairly easy to implement these two features here. Considering the consistency between the client and server side versions, I just did fixed matching and case-insensitive searching.

I'll add a couple of demos later.