Support data-mx[-bg]-color attributes

[dkasak]

It seems the Android client doesn't recognize the data-mx[-bg]-color attributes on font and span tags while the web/desktop Riot does. Furthermore, in web Riot, wrapping the text in <span style="background-color: ...">...</span> doesn't set the background color anymore (while it does in the Android client). Judging by this discussion, the data-mx[-bg]-color attributes seem to be the way to go.

Therefore, it seems that support for these attributes should be added to the Android client as well.

[dkasak]

This is now in the spec so bug and parity-with-web labels seem appropriate.

[bmarty]

bug label is reserved for bug in implemented features, not for missing feature