Improved variable parsing in YAML files

[caronc]

Description:

Related issue (if applicable): #1087

Better parsing of YAML files that contain globals parsed by Apprise itself (such as user, pass, verify, etc)

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@1087-notify-email-variables

# Prepare a config file:
cat _EOF > test.yaml
 urls:
      - mailtos://alt.lan/:
         - user: testuser@alt.lan
           pass: xxxxXXXxxx
           smtp: smtp.alt.lan
           to: alteriks@alt.lan
_EOF

# Test out the changes with the following command:
apprise -vvvv  -t "Test Title" -b "Test Message" -c test.yaml

# Verify the debug logs correctly parse the pass variable 

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.20%. Comparing base (f55f691) to head (93370d4).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
- Coverage   99.27%   99.20%   -0.08%     
==========================================
  Files         137      137              
  Lines       17888    17901      +13     
  Branches     3652     3657       +5     
==========================================
  Hits        17758    17758              
- Misses        121      133      +12     
- Partials        9       10       +1     

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