Allow multiple modal widths

[mrozbarry]

It would be nice to be able to set a small/medium/large width on modals, or allow some style overrides for the width.

[gdotdesign]

Modals fit their contents, they have only max-width: 80vw and max-height: 80vh set as constraints which can be removed.

We can add optional width and height in it's ViewModel like so:

type alias ViewModel msg =
  { contents : List (Html.Html msg)
  , footer : List (Html.Html msg)
  , address : (Msg -> msg)
  , title : String

  , width : Maybe Float
  , height : Maybe Float
  }

But I don't know if it's justified enough to do so, because the width and height of ui-modal-wrapper can be set easily with CSS.