Support for t2bot.io webhooks for Matrix

[voidmann]

Description:

Was described in issue #80 but never worked with webhook.t2bot.io. They do now.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov-io]

Codecov Report

Merging #189 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #189   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          71     71           
  Lines        8323   8351   +28     
  Branches     1333   1341    +8     
=====================================
+ Hits         8323   8351   +28

Impacted Files	Coverage Δ
apprise/plugins/NotifyMatrix.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 408810d...7d7f83b. Read the comment docs.

[caronc]

Thank you!

Changes look awesome!

Edit: I updated the wiki to reflect your merge request.