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

• [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@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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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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