Performance fix - unnecessary rune convertion on --ansi

> time cat /tmp/list | fzf-0.10.1-darwin_amd64 --ansi -fqwerty > /dev/null

    real    0m4.364s
    user    0m8.231s
    sys     0m0.820s

    > time cat /tmp/list | fzf --ansi -fqwerty > /dev/null

    real    0m4.624s
    user    0m5.755s
    sys     0m0.732s

src/util/util.go

@@ -6,6 +6,7 @@ import "C"
 import (
 	"os"
 	"time"
+	"unicode/utf8"
 )
 
 // Max returns the largest integer
@@ -88,3 +89,18 @@ func TrimRight(runes []rune) []rune {
 	}
 	return runes[0 : i+1]
 }
+
+func BytesToRunes(bytea []byte) []rune {
+	runes := make([]rune, 0, len(bytea))
+	for i := 0; i < len(bytea); {
+		if bytea[i] < utf8.RuneSelf {
+			runes = append(runes, rune(bytea[i]))
+			i++
+		} else {
+			r, sz := utf8.DecodeRune(bytea[i:])
+			i += sz
+			runes = append(runes, r)
+		}
+	}
+	return runes
+}