Added Matrix Attachment support

[caronc]

Description:

Related issue (if applicable): #795 and #356

Added Matrix Attachment support

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

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@795-matrix-attachment-support

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  "matrix://credentials" \
 --attach=/path/to/file

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c56d4c5) 99.52% compared to head (665f5ab) 99.52%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #921   +/-   ##
=======================================
  Coverage   99.52%   99.52%           
=======================================
  Files         122      122           
  Lines       16235    16290   +55     
  Branches     3309     3327   +18     
=======================================
+ Hits        16158    16213   +55     
  Misses         68       68           
  Partials        9        9           

Files Changed	Coverage Δ
apprise/plugins/NotifyMatrix.py	100.00% <100.00%> (ø)

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