Add a "context" setting to the popup module

In our current project we needed to change the element the popup is appended to. We did that so that we may wrap semantic ui CSS selectors with a prefix :

    <body>
      ...
      <div class="custom-prefix modal reference"></div>
    </body>

manifest.scss :

    .custom-prefix{
      ...
      @import "third-parties/semantic/modules/popup";
      ...
    }

This use case is quite specific but I guess it could give more flexibility to the popup module. I mimiced the the way the "context" property of the modal module is used.