create colored teletext subs

no dedicated configuration setting, since if color is not needed/wanted,
~all playback software that support colored subs allow to override/ignore it.
And playback software that doesn't support colored subs just will typically
ignore the font-tags.

Rather than the color, the start = endtime problem
https://tvheadend.org/issues/756 
is more problematic regarding compatibilty with playback software