Add Ignore configuration value

Ignoring a set of characters allows for annotations on the map that can
be used by the DM (and these annotations will not interfere with future
versions that support additional map symbols).

draw.go

@@ -16,6 +16,7 @@ type MapData struct {
 	Light               uint8   // wall at LightAngle angle this (max) amplification
 	LightAngle          int16   // in degrees
 	Output              string  // Output filename (must end in .png)
+	Ignore              string  // Characters to ignore (default 1234567890)
 	line                []string
 	pic                 image.Image
 }
@@ -30,6 +31,7 @@ func (m *MapData) defaults() {
 	m.ShadowWidth = 0.2
 	m.Light = 15
 	m.LightAngle = 10
+	m.Ignore = "1234567890"
 }
 
 type fromType struct {

read.go

@@ -70,6 +70,13 @@ func read(fn string) (*MapData, error) {
 }
 
 func (m *MapData) rectangle() {
+	// Trim ignored characters
+	for _, c := range m.Ignore {
+		for i := range m.line {
+			m.line[i] = strings.ReplaceAll(m.line[i], string(c), " ")
+		}
+	}
+
 	// trim leading empty lines
 	for len(m.line) > 0 && len(strings.TrimSpace(m.line[0])) == 0 {
 		m.line = m.line[1:]