feat(rxStatusColumn): Add support for coloured status columns

This provides `rx-status-header` and `rx-status-column` attribute
directives, used to modify `<th>` and `<td>` elements for the purpose
of showing coloured statuses in one column of a table.

You can define mappings to go from your own internal status values
to the four currently provided by the framework.

Closes #688
I've filed an issue https://github.com/rackerlabs/encore-ui/issues/698

I took a quick run at it, but it wasn't working, and I'd like to release today.