Addtional underline support

[SimonRev]

Many terminals today support more than just plain underline, they support

• Double underline
• Overline
• Styled underlines such as squiggles, dashed and dotted
• Underline color distinct from foreground color

I would be interested in adding support for some of these features.

I have taken a quick start at some code. Double underline and Overline are pretty trivial. Underline color touches a bit more code since it implies an additional Color property within the Style class.

I think the biggest technical challenge is that Windows Terminal requires using colon instead of semicolon as the subcommand seperator when setting underline color. Otherwise the underline color is just sequence 58:2:r:g:b or 58:5:c. (Windows terminal requires colons. Gnome terminal accepts either colons or semicolons). Both gnome terminal and Windows terminal require colons when specifying a stylized underline. This might require some additional enrichments to know which terminals support these features.

This is something that I would be willing to implement, including documentation and tests, but am curious if it would be accepted. I would propose doing it in 2 parts: First the double underline and overline which are simpler and then adding the full underline style/color support.