Apprise configuration files to support 'include' keyword

[caronc]

💡 The Idea
I was just thinking that it might be useful to support the keyword include in both the YAML and TEXT Apprise configuration files. For text files it would look like:

include http://localhost/path/to/more/configuration/entries
# No limit to the number of includes you specify
include file:///path/to/another/configuration/file/on/your/system

YAML configuration files would be similar, it would just be it's own section:

include:
   - http://localhost/path/to/more/configuration/entries
   - file:///path/to/another/configuration/file/on/your/system

The idea here is people who use the default (always iincluded configuration files that the CLI offers), they can use this entry as a way to have the CLI automatically fetch from a remote server.

Things to consider:

• A file could include a file/website to which it also has an include statement.
  • recursion of how many includes take place should be controlled (default it to 1 recursive request) but allow users to increase this if they wish.
  • consider that an include could effectively include itself (solved by controlling recursion above).
• By default people who aren't using the apprise CLI would not have to worry as the include feature would be ignored by default unless they explicitly turned it on.
• Those who want to turn the feature on, would just need to include a keyword argument with the AppriseConfig() object to do so. I was thinking of making a variable called recursion which by default is set to 0 (or None). Setting this to a positive integer however would define how many times an include could be called from within a configuration already included. e.g.:

  import apprise
  
  # Allow up to 3 levels of recursion (over-riding default value of zero)
  config = apprise.AppriseConfig(recursion=3)
  
  # Now load our configuration
  config.add('/path/to/file/that/includes/config/entries/and/or/include/entries/too.txt')

• include would respect cache and re-retrieve content if required to do so

🔒 Security

• Relative paths specified in file:// based configurations will include relative to the configuration file it was read from.
• It will not be possible to include file:// based configuration if the request to do so came from a non-file:// based one (such as http://). Only a file:// based configuration can include another file:// based one.
• memory:// based configurations can't include file:// based either on it's own. However developers can set insecure_includes=True when calling Apprise.add_config() to over-ride this.

🔨 Breaking Feature
Nothing would break per say but this introduces some security issues that i think having the include feature disabled by default will solve.

[caronc]

code is now merged