Add support for features released in semantic UI 2.4

[abhinavk96]

I am unable to use the clearable property of ui-dropdown which was released in version 2.4 of Semantic UI. Any workarounds would be appreciated, thanks!

[wuarmin]

I recommend using ember-power-select.
I could share my styling scss-vars, if you want.

[abhinavk96]

Thanks @wuarmin