better custom email detection for `from_addr` and `to` if not specified

[caronc]

Description:

Related issue (if applicable): #1040

Custom email servers matched to a template now better detect the from_addr and to if not specified. This significantly shortens the URL required.

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@1040-mailto-better-email-detection

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  "mailto://fastmail.com?user=username@customdomain.com&pass=password123"

[caronc]

@taestell : The added unit tests seem to prove it's working, but i would love your feedback!

[codecov-commenter]

Codecov Report

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

Comparison is base (010be0b) 99.27% compared to head (340ead6) 99.27%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1066 +/- ## ======================================= Coverage 99.27% 99.27% ======================================= Files 136 136 Lines 17738 17739 +1 Branches 3618 3618 ======================================= + Hits 17610 17611 +1 Misses 119 119 Partials 9 9 ```

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[taestell]

Confirmed to be working using my custom Fastmail domain and specifying only the user and pass params. Great work, thanks!