Plugin to select2 (https://github.com/select2/select2) allowing multiple selection while retain old good dropdown.
Some of developers (including me) thinks that multiple selection in select2 is ugly due to oversized display of selected items.
With small effort it can be replaced by normal drop down list with checkboxes.
Docs (comming soon)
Take look into https://jsfiddle.net/wasikuss/7ak9skbb/ to learn how to use it.