[enhancement] more free (as in speech) and essential notifiers

[DJCrashdummy]

System information

• Have I written custom code (as opposed to using zenbot vanilla): no
• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Debian stretch
• Zenbot version (commit ref, or version): 23ae02e
• Zenbot branch: master
• NodeJS version: 10.x
• Python version (when using a python script): no
• Exact command to reproduce (include everything): not relevant
• Did I make any changes to conf-sample.js?: no

Describe the problem

well... i'm thankful for the XMPP notifier, but i am sorely missing 2 other free networks/protocols: email and matrix!

IMHO these two protocols are essential, because email is by far the most widely used one and matrix gains more and more use and would perhaps be the easiest one with a real and trustworthy E2E encryption (even for more than one recipient).