support Content-Type and Content-Transfer-Encoding access for each parsed email parts

[ShawnXU0208]

--000000000000d6c5d20640658b56
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

these two headers seems to be ignored completely.

    parser.on('data', async (data) => {
      if (data.type === 'text') {
        console.log(data);
        mailobj.emails = parseEmailText(data.text);
      }
    });

the returned data doesn't contain these two