Added LESS file for Free Bootswatch's Flatly theme.

I don't know if you would like to maintain aditional LESS files for Bootswatch themes.
But I've tweaked the `selectize.bootstrap3.less` file a bit so it would look better using Flatly theme.
The lines I've edited are marked with comments.

Essentialy:
* `@selectize-border` changed to `2px`
* Added transition to `.selectize-input`
* Removed box-shadow of `.selectize-input .focus`
* Changed `.selectize-dropdown .active` a bit.