Persistent Storage

[caronc]

Description:

Related issue (if applicable): #749

Persistent Storage - WIP

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@749-persistent-storage

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.33206% with 7 lines in your changes missing coverage. Please review.

Project coverage is 99.19%. Comparing base (6733953) to head (dab9fa4). Report is 1 commits behind head on master.

Files	Patch %	Lines
apprise/persistent_store.py	99.64%	2 Missing :warning:
apprise/url.py	96.42%	1 Missing and 1 partial :warning:
apprise/plugins/dbus.py	66.66%	1 Missing :warning:
apprise/plugins/email.py	66.66%	1 Missing :warning:
apprise/plugins/syslog.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1131 +/- ## ========================================== - Coverage 99.28% 99.19% -0.10% ========================================== Files 142 143 +1 Lines 18440 19419 +979 Branches 3763 3771 +8 ========================================== + Hits 18309 19263 +954 - Misses 121 142 +21 - Partials 10 14 +4 ```

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