Slack supports <@userid> in message body

[caronc]

Description:

Related issue (if applicable): #849

Added support for <@userid> and <@userid|desc> to slack payload

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-userid-in-body

# Test out the changes with the following command:
apprise -t "Test Title" -b "Hi <@U1ZQL9N3Y>!" \
  "slack://credentials/"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.28%. Comparing base (da14b3a) to head (1ef401c).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1134   +/-   ##
=======================================
  Coverage   99.28%   99.28%           
=======================================
  Files         140      140           
  Lines       18229    18234    +5     
  Branches     3728     3729    +1     
=======================================
+ Hits        18098    18103    +5     
  Misses        121      121           
  Partials       10       10           

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