Add DefaultUnderline font to ChatScheme.res to allow third party programmers to add clickable links to chat

This is what my code looks like with the vanilla ChatScheme.res:

![gay]http://peniscorp.com/stuff/Garry's_Mod_2013-08-23_23-26-28.png

This is what it looks like with that ChatScheme.res:

![lessgay]http://peniscorp.com/stuff/Garry's_Mod_2013-08-24_00-16-38.png