This adds support for double underlines using the colon separated escape
sequence `CSI 4 : 2 m`.
Alacritty will now also always fallback to the normal underline in case
any of the other underlines like the undercurl are specified. The escape
sequence `CSI 4 : 0 m` can now be used to clear all underlines.
Some terminals support `CSI 21 m` for double underline, but since
Alacritty already uses that as cancel bold which is a little more
consistent, that behavior has not changed. So the colon separated
variant must be used.
