Improved markdown to html conversions

[caronc]

Description:

Related issue (if applicable): #1084

Better MARKDOWN to HTML Conversion

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@1084-email-markdown-formatting

cat << _EOF > test_markdown
## Some Heading

With Data:

- Foo
- Bar

## Table Support Added Too

First Header  | Second Header
------------- | -------------
Content Cell  | Content Cell
Content Cell  | Content Cell
_EOF

# Test out the changes with the following command:
apprise -vvv -t "Test Title" -b "$(<test_markdown)" \
    -i markdown \
    "mailtos://credentials"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.27%. Comparing base (2c5341a) to head (06eca3e).

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1089   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files         137      137           
  Lines       17901    17901           
  Branches     3657     3657           
=======================================
  Hits        17771    17771           
  Misses        121      121           
  Partials        9        9           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.