Fix two byte character issue

The colored string contains the context of the input string rune by rune
with each rune having a respective color setting. However, the parsing
transferred the input string byte by byte into the colored string. There
is a problem for inputs that contain multi byte characters. Copying them
required that the two byte characters are processed at once.

Switch copy for look to work on a rune slice rather than bytes.

parse.go

@@ -68,8 +68,8 @@ func ParseString(input string, opts ...ParseOption) (*String, error) {
 		fullMatchStart, fullMatchEnd := submatch[0], submatch[1]
 		settingsStart, settingsEnd := submatch[2], submatch[3]
 
-		for i := pointer; i < fullMatchStart; i++ {
-			result = append(result, ColoredRune{rune(input[i]), current})
+		for _, r := range input[pointer:fullMatchStart] {
+			result = append(result, ColoredRune{r, current})
 		}
 
 		current, err = parseSelectGraphicRenditionEscapeSequence(input[settingsStart:settingsEnd])
@@ -81,8 +81,8 @@ func ParseString(input string, opts ...ParseOption) (*String, error) {
 	}
 
 	// Flush the remaining input string part into the result
-	for i := pointer; i < len(input); i++ {
-		result = append(result, ColoredRune{rune(input[i]), current})
+	for _, r := range input[pointer:] {
+		result = append(result, ColoredRune{r, current})
 	}
 
 	// Process optional parser options

render.go

@@ -54,7 +54,7 @@ func (s String) String() string {
 			current = coloredRune.Settings
 		}
 
-		_ = buffer.WriteByte(byte(coloredRune.Symbol))
+		_, _ = buffer.WriteRune(coloredRune.Symbol)
 	}
 
 	// Make sure to finish with a reset escape sequence