*
@method hiragana([all] = true)
@returns String
@short Converts katakana into hiragana.
@extra If [all] is false, only full-width katakana will be converted.
@example

'カタカナ'.hiragana()   -> 'かたかな'
'コンニチハ'.hiragana() -> 'こんにちは'
'ｶﾀｶﾅ'.hiragana()       -> 'かたかな'
'ｶﾀｶﾅ'.hiragana(false)  -> 'ｶﾀｶﾅ'

*
function(all) {
      var str = this;
      if(all !== false) {
        str = str.zenkaku('k');
      }
      return str.replace(/[\u30A1-\u30F6]/g, function(c) {
        return c.shift(-96);
      });
    }