Discord now supports href= (and alias url=) arguments

[caronc]

Description:

Related issue (if applicable): #905

Parameter Breakdown

Variable	Required	Description
href	No	Identify a URL the title should link to when posting the Discord Notification. This forces the post into markdown format in order to leverage the embeds section of Discord. You can also use url= as an alias of this as well.

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@905-discord-enhancements

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  "discord://credentials/?href=https://github.com/caronc/apprise"

# Another way of writing that is:
apprise -t "Test Title" -b "Test Message" \
  "discord://credentials/?url=https://github.com/caronc/apprise"

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (207db69) 99.52% compared to head (a7e8273) 99.52%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #927   +/-   ##
=======================================
  Coverage   99.52%   99.52%           
=======================================
  Files         122      122           
  Lines       16290    16301   +11     
  Branches     3327     3331    +4     
=======================================
+ Hits        16213    16224   +11     
  Misses         68       68           
  Partials        9        9           

Files Changed	Coverage Δ
apprise/plugins/NotifyDiscord.py	100.00% <100.00%> (ø)

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