Vertical scroll indicators

[ahmacleod]

Vertical analogue of #814. Shows horizontal shadows when it is possible to scroll up or down. Respects any header or footer rows.

Extends #853

With scrollbars-always-on:

Usage:

<EmberTable as |t|>
  <t.head
    @columns={{columns}}
    @scrollIndicators="all"
  />
  <t.body @rows={{rows}} />
</EmberTable>

Possible string values for scrollIndicators are all, horizontal, vertical, and none. Boolean values true and false are aliased to all and none, respectively.

[twokul]

Looks good to me! Could you add a gif to the PR description and maybe a code snippet (copy-paste from the one in the docs)?