Improve Livesearch Performance

[andrews05]

When working with large lists, livesearch can start to show a little lag when typing. I've reworked the input listener to improve the performance. The gains are primarily from reducing the number of queries that are made. In my test case, this code runs at least 10x faster than before.
This also fixes a couple of edge cases where a divider wouldn't show/hide correctly.

[andrews05]

Minified files removed as per updated guidelines.
Keen to hear any comments on this as it makes a real difference to how responsive it feels in some of the menus I'm using.

[poziomek]

1. List with 3600 entries, both: values and displayed text are strings. Changes from this commit help us to reduce search latency (during typing). Great work.
2. I've took source codes directly from commit (bootstrap-select.js file from commit window) and it seems that works.
   • However when I took minified version from dist folder (https://github.com/andrews05/bootstrap-select/blob/7143fcba4ed19f1d539a3460863bdae1ffcd0ced/dist/js/bootstrap-select.min.js) then error raises (script doesn't compile) in browser.

[caseyjhol]

Will try to get this implemented ASAP.

[poziomek]

Thank you!

[caseyjhol]

This also addresses #1275.

[andrews05]

Thanks for merging this! Glad to hear I'm not the only one working with stupidly large lists.