Add support for a third color in face definition that controls
the underline and a 'c' attribute for curly underline (that takes
precedence over 'u' if both are specified)
Allow empty colors to mean default, so that `,,red+u` means the
same as `default,default,red+u`
Fixes#4138
When a region calls the regex highlighter, it is incorrect to share
the regex cache as it means we can get matches that span multiple
regions.
Fixes#3041