Add Dropdown support

[licatajustin]

Addresses #34.

Dropdown support is 100% data-attribute driven and is based on the same HTML implementation of Bootstrap dropdowns.

<div class="dropdown">
  <button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    Dropdown button
  </button>
  <div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Another action</a>
    <a class="dropdown-item" href="#">Something else here</a>
  </div>
</div>

[afomera]

Works as intended. Tested on my apps with jQuery, and Turbolinks 5.