YAML extended configuration support for 'include' keyword

[caronc]

Description:

Related issue (if applicable): #226

Just adding extended support to allow for comma separated include entries for YAML files only. To keep TEXT configuration files clean, they should still only have one include URL per statement.

This new Pull Request extends the existing YAML include support to additionally allows this:

# comma separated
include: http://localhost:8080/notify/apprise, http://localhost/apprise/cfg

... and this:

# new entry, but not bulleted (which is already supported)
# entries are still honoured and parsed accordingly to avoid end user
# confusion.
include:
   http://localhost:8080/notify/apprise
   http://localhost/apprise/cfg01
   http://localhost/apprise/cfg02
   http://localhost/apprise/cfg03

... and finally this too:

# a mix of supported bullets and comma separated URLs.  This
# looks unclean for what the intentions are, but there is no need to
# yell at a user to change it when it's still very parse-able.
include:
   - http://localhost:8080/notify/apprise, http://localhost/apprise/cfg01
     http://localhost/apprise/cfg02
   - http://localhost/apprise/cfg03

This merge request contains test cases to support the 3 examples above.

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

[codecov-commenter]

Codecov Report

Merging #284 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #284   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           83        83           
  Lines         9927      9928    +1     
  Branches      1611      1611           
=========================================
+ Hits          9927      9928    +1     

Impacted Files	Coverage Δ
apprise/config/ConfigBase.py	100.00% <100.00%> (ø)

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