Use more face properties when updating output

[pkryger]

When difftastic output is customized using user defined faces only foreground and background properties are used. This is to update difftastic--ansi-color-add-background functions family to support more properties from user defined faces.

A few properties that match ansi-color (i.e, bold, underline) could be re-use respective ansi-color faces, for others, need to investigate how font-lock is doing that.