Add support .yaml files via CLI

[caronc]

Description:

Related issue (if applicable): #1069

Allow Apprise to support .yaml files in addition to .yml

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@support-yaml-files

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.27%. Comparing base (26d8e45) to head (d1c7dc6).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1073   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files         136      136           
  Lines       17753    17753           
  Branches     3623     3623           
=======================================
  Hits        17625    17625           
  Misses        119      119           
  Partials        9        9           

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