Improved markdown to html conversions

[caronc]

Description:

Related issue (if applicable): #1084

Better MARKDOWN to HTML Conversion

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] 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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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 ```

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