Refactored (Split and Truncate) Overflow Engine

[caronc]

Description:

Related issue (if applicable): #845

A massive refactoring of the Overflow Engine handling the truncate and split functionality. A more dynamic and clean presentation can be adapted per upstream service.

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@845-overflow-continued-improvements

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  <apprise url related to ticket>?overflow=split

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f3c699a) 99.27% compared to head (11ba9e9) 99.27%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1038   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files         135      135           
  Lines       17574    17600   +26     
  Branches     3588     3592    +4     
=======================================
+ Hits        17446    17472   +26     
  Misses        119      119           
  Partials        9        9           

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