supporting text does not show on horizontal cards

[rapropos]

<mdc-card>
<mdc-card-horizontal>
<mdc-card-supporting-text>in oblivion</mdc-card-supporting-text>
</mdc-card-horizontal>
</mdc-card>

Taking the <mdc-card-horizontal> out, the supporting text appears. Strangely, I couldn't even find the element in the DOM in the horizontal case. If there is some reason that this can't work, then I would suggest at least putting a note in the documentation telling people not to try.

[trimox]

@rapropos Yeah, mdc-card-horizontal selector is a bit too strict on content acceptance. Working on a PR..