mqtt:// support for publishing retain flag

[caronc]

Description:

Related issue (if applicable): #799

mqtt:// URL now supports retain=yes as a kwarg parameter supported. It defaults to no to avoid obstructing previous behavior

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@799-mqtt-retain-flag

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" "mqtt://host/topic/?retain=yes"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.35%. Comparing base (827db52) to head (1cf8f17).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1185   +/-   ##
=======================================
  Coverage   99.35%   99.35%           
=======================================
  Files         146      146           
  Lines       20176    20179    +3     
  Branches     3966     3967    +1     
=======================================
+ Hits        20045    20048    +3     
  Misses        121      121           
  Partials       10       10           

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