ヂ is not handled correctly #28

The (rare) katakana 'ヂ' is not handled correctly in KanaToRomaji.

Test program:

package main

import (
    "fmt"

    "github.com/gojp/kana"
)

func main() {
	var test = "アイウエオダヂヅデド"
	fmt.Printf("Kana results: %s -> %s", test, kana.KanaToRomaji(test))
}

Output:
Kana results: アイウエオダヂヅデド -> aiueodaヂdudedo

Expected output:
Kana results: アイウエオダヂヅデド -> aiueodadidudedo

If you are interested, I can make a PR to fix it.