Add dark mode & perspective to Board.Draw()

This commit adds Board.Draw2(perspective Color, darkMode bool). This
is similar to Board.Draw() except that it allows the caller to specify
from which perspective to display and whether the display should be
reversed colors. (In a terminal window configured with white foreground
text and a black background, the current Board.Draw() displays the
opposite color).

(cherry picked from commit 2c91017)

board.go

@@ -108,6 +108,22 @@ func (b *Board) Transpose() *Board {
 
 // Draw returns visual representation of the board useful for debugging.
 func (b *Board) Draw() string {
+	return b.drawForWhite(false)
+}
+
+// Draw2 returns visual representation of the board useful for debugging.
+// It is similar to Draw() except allows the caller to specify perspective
+// and dark mode options
+func (b *Board) Draw2(perspective Color, darkMode bool) string {
+	if perspective == Black {
+		return b.drawForBlack(darkMode)
+	} // else
+
+	return b.drawForWhite(darkMode)
+}
+
+// drawForWhite returns visual representation of the board from white's perspective
+func (b *Board) drawForWhite(darkMode bool) string {
 	s := "\n A B C D E F G H\n"
 	for r := 7; r >= 0; r-- {
 		s += Rank(r).String()
@@ -116,7 +132,34 @@ func (b *Board) Draw() string {
 			if p == NoPiece {
 				s += "-"
 			} else {
-				s += p.String()
+				if darkMode {
+					s += p.DarkString()
+				} else {
+					s += p.String()
+				}
+			}
+			s += " "
+		}
+		s += "\n"
+	}
+	return s
+}
+
+// drawForBlack returns visual representation of the board from black's perspective
+func (b *Board) drawForBlack(darkMode bool) string {
+	s := "\n H G F E D C B A\n"
+	for r := 0; r <= 7; r++ {
+		s += Rank(r).String()
+		for f := numOfSquaresInRow - 1; f >= 0; f-- {
+			p := b.Piece(NewSquare(File(f), Rank(r)))
+			if p == NoPiece {
+				s += "-"
+			} else {
+				if darkMode {
+					s += p.DarkString()
+				} else {
+					s += p.String()
+				}
 			}
 			s += " "
 		}

piece.go

@@ -174,8 +174,13 @@ func (p Piece) String() string {
 	return pieceUnicodes[int(p)]
 }
 
+func (p Piece) DarkString() string {
+	return pieceDarkUnicodes[int(p)]
+}
+
 var (
-	pieceUnicodes = []string{" ", "♔", "♕", "♖", "♗", "♘", "♙", "♚", "♛", "♜", "♝", "♞", "♟"}
+	pieceUnicodes     = []string{" ", "♔", "♕", "♖", "♗", "♘", "♙", "♚", "♛", "♜", "♝", "♞", "♟"}
+	pieceDarkUnicodes = []string{" ", "♚", "♛", "♜", "♝", "♞", "♟", "♔", "♕", "♖", "♗", "♘", "♙"}
 )
 
 func (p Piece) getFENChar() string {