The product isn't quite finish at the time of posting, because I want to add lots of more features to it. This will run w3m on your mail if it matches one of the following conditions:
* The Content-Type of the mail is text/html
* The Content-Type is multipart/alternative and the first part is text/html
* The Content-Type is multipart/alternative and the second part is text/html (Useful when mail is transmitted in both text/plain and text/html