Add TrueColorTags option

This controls whether color tags should render as the specific colors
defined by tcell, or as the colors defined by the user's terminal
configuration (the default).

Relates to #48.

util.go

@@ -12,6 +12,11 @@ import (
 	"github.com/rivo/uniseg"
 )
 
+// TrueColorTags is a flag which controls whether color tags should render as
+// the specific colors defined by tcell, or as the colors defined by the user's
+// terminal configuration (the default).
+var TrueColorTags = false
+
 // ColorUnset represents an unset color. This is necessary because the zero
 // value of color, ColorDefault, results in default terminal colors.
 var ColorUnset = tcell.ColorSpecial | 108
@@ -180,14 +185,22 @@ func overlayStyle(background tcell.Color, defaultStyle tcell.Style, fgColor, bgC
 		if fgColor == "-" {
 			style = style.Foreground(defFg)
 		} else {
-			style = style.Foreground(tcell.GetColor(fgColor).TrueColor())
+			c := tcell.GetColor(fgColor)
+			if TrueColorTags {
+				c = c.TrueColor()
+			}
+			style = style.Foreground(c)
 		}
 	}
 
 	if bgColor == "-" || bgColor == "" && defBg != tcell.ColorDefault {
 		style = style.Background(defBg)
 	} else if bgColor != "" {
-		style = style.Background(tcell.GetColor(bgColor).TrueColor())
+		c := tcell.GetColor(bgColor)
+		if TrueColorTags {
+			c = c.TrueColor()
+		}
+		style = style.Background(c)
 	}
 
 	if attributes == "-" {