utf8 support

Unicode characters are getting mangled for me. Are you able to replicate?

Sample program:
```
package main

import (
	"os"
	"strings"

	"github.com/arsham/rainbow/rainbow"
)

func main() {

	testStr := "🏧 --- ✂"

	rb := rainbow.Light{
		Reader: strings.NewReader(testStr),
		Writer: os.Stdout,
	}
	rb.Paint()
}
```