Email testing

httpx supports to send email by SMTP.

### send an email by Gmail
//@name mail
MAIL mailto:[email protected]
Host: tls://smtp.gmail.com:587
Authorization: Basic [email protected]:google_app_password
From: [email protected]
Subject: e-nice to meet you
Content-Type: text/plain

Hi Master:
  this is testing email.

Best regards
Yours sincerely Zombie

Attentions:

• Gmail: please use App Password from https://support.google.com/accounts/answer/185833?p=InvalidSecondFactor