HTML to Markdown support

[caronc]

Description

Related issue (if applicable): #1600

This implements a proper HTML-to-Markdown converter for Apprise, completing the work started in PR #932.

This is also a starting marker to eventually handle the #1600 which extends beyond this.

Element	Output
<h1> … <h6>	# … ###### prefixed line
<b>, <strong>	**text**
<em>, <i>	*text*
<code>	`text` (inline, CR preserved, no escape)
<pre>, <samp>	``` fenced block
<a href="…">	[text](url)
<a> (no href)	plain text
<blockquote>	> text
<li>	- text
<hr>	---
<br>	literal newline
*, #, ` in body text	escaped as \*, \#, \`
<script>, <style>, etc.	suppressed

Checklist

• Documentation ticket created (if applicable): n/a
• The change is tested and works locally.
• No commented-out code in this PR.
• No lint errors (use tox -e lint and optionally tox -e format).
• Test coverage added or updated (use tox -e qa).

Testing

Anyone can help test as follows:

# Create a virtual environment
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@1600-html-to-markdown

# If you have cloned the branch and have tox available to you:
tox -e apprise -- -t "Test Title" -b "Test Message" \
    <apprise url that converts HTML to markdown, e.g. gotify://...>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ec565cd) to head (a5a953a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master     #1638    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          212       212            
  Lines        30089     30257   +168     
  Branches      5015      5069    +54     
==========================================
+ Hits         30089     30257   +168     

Files with missing lines	Coverage Δ
apprise/conversion.py	100.00% <100.00%> (ø)
apprise/plugins/telegram.py	100.00% <ø> (ø)

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