Added delay= switch to aprs:// plugin

[caronc]

Description:

Related issue (if applicable): #1105

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@1105-aprs-delay-switch

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  aprs://credentials?delay=2.0

[codecov-commenter]

Codecov Report

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

Project coverage is 99.27%. Comparing base (cced5ed) to head (f0322e4). Report is 3 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1107 +/- ## ======================================= Coverage 99.27% 99.27% ======================================= Files 139 139 Lines 18029 18045 +16 Branches 3680 3685 +5 ======================================= + Hits 17899 17915 +16 Misses 121 121 Partials 9 9 ```

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