Slack <!channel|desc> & <!channel> support added

[caronc]

Description:

Related issue (if applicable): #849

Support <!channel> and <!channel|desc> directives

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@849-slack-channel-mention-support

# Test out the changes with the following command:
apprise -t "Test Title" -b "<!channel>" \
   "slack://credentials"

[caronc]

@lanrat: i think i got this for you; would you please give it a try and let me know how it goes?

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2057107) 100.00% compared to head (ce1f6b3) 100.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #856   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          118       118           
  Lines        15366     15378   +12     
  Branches      3153      3155    +2     
=========================================
+ Hits         15366     15378   +12     

Impacted Files	Coverage Δ
apprise/plugins/NotifySlack.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[lanrat]

I'm happy to help test.

Is the best way to pull this PR directly?

[caronc]

Anyway works, as long as you use this branch.

Honestly, I'm pretty sure it will work as i even added a test case to verify it. But sometimes for the peace of mind, It's nice hearing from someone else confirming it.

It's my understanding that the <@user> is working fine?

[lanrat]

User mentions <@user> already worked.

[lanrat]

I'm having some difficulty getting a built to test with.

I use Docker for my setup. I cloned your branch and built a docker image using the Dockerfile.py310 file, but it seems to not be the same type of image you have published to the Docker Hub. I tried poking around in the docker image to find a way to start the apprise service, and looking for development documentation in the repo and in your wiki, but could not find anything for Docker images, just plugin development. Is there something I'm missing here?

[caronc]

No worries, Merging after testing myself.

[lanrat]

Can we get a new release of apprise that includes this?