Add option for minimum selection

[shirishparsekar]

Summary
Adds an option (minCount) for specifying minimum selection.

Example

    $('.my-dropdown').dropdown({
      minCount: 2,
      limitCount: 5,
    });

If the user unselects an option after selecting 2 options, he will be shown an alert saying he needs to select minimum 2. However, the user will not be restricted from unselecting an option. We are only displaying an alert.