.conf configuration file support added to CLI + doc ambiguity cleanup

Description:

Related issue (if applicable): #1076

Eliminate confusion of the configuration file /etc/apprise by removing this entirely from the documentation (but not from the source code - keeping it backwards compatible).

Adding .conf extension support and improving the documentation to focus around this. Hopefully this will make it a bit more clear for those using the software. Apprise very much lacks good documentation. This issue is a segway into trying to fulfill #869 someday.

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@1076-cli-conf-support

# Now Additionally Supports:
#  ~/.apprise.conf
#  ~/.config/apprise.conf
#  /etc/apprise.conf

# also
#  ~/.apprise/apprise/apprise.conf
#  ~/.config/apprise/apprise.conf
#  /etc/apprise/apprise.conf

apprise -t "Test Title" -b "Test Message" \
  <apprise url related to ticket>

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.27%. Comparing base (d5cbab1) to head (9d17075).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1080 +/- ## ======================================= Coverage 99.27% 99.27% ======================================= Files 136 136 Lines 17752 17752 Branches 3622 3622 ======================================= Hits 17624 17624 Misses 119 119 Partials 9 9 ```

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caronc / apprise