Add support .yaml files via CLI

[caronc]

Description:

Related issue (if applicable): #1069

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

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@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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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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