Discord user/role ping/notification support added

[caronc]

Description:

Related issue (if applicable): #989

Added PING support to Discord related notifications. Syntax supported is as follows:

• user: <@123>
• role: <@&456>
• tag: @everyone

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@989-discord-notifications

# Test out the changes with the following command:
apprise -vv -b "@everyone and @admin, wake and meet our new user <@123>; <@&456>" \
  "discord://credentials"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c499762) 99.51% compared to head (ee70d84) 99.51%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1004   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files         125      125           
  Lines       16760    16778   +18     
  Branches     3434     3440    +6     
=======================================
+ Hits        16679    16697   +18     
  Misses         70       70           
  Partials       11       11           

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