APPRISE_ALLOW_SERVICES and APPRISE_DENY_SERVICES globals added

[caronc]

Description:

Related issue (if applicable): n/a

Adds 2 new global variables that allow users to disable services they don't want people using. The filters work in 2 ways:

1. APPRISE_ALLOW_SERVICES: A comma separated list that defines any services (by it's schema) you want to exclusively allow (denying everything else).
2. APPRISE_DENY_SERVICES: A comma separated list that defines any services (by it's schema) deny (without touching other services). By default this is pre-configured to block local services: windows,dbus,gnome,macosx,syslog

Other things worth mentioning:

• If both variables are defined, APPRISE_ALLOW_SERVICES will always take priority (and APPRISE_DENY_SERVICES will be ignored).
• It's not case sensitive. So MAILTO will still match gracefully against the mailto schema.
• Your disabling is happening at a global scale; so if you specify mailtos, you're still disabling the mailto (and vs versa); so there is no need to specify both.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• tests added

[codecov-commenter]

Codecov Report

Merging #64 (025f84b) into master (efe88a5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #64   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          418       466   +48     
=========================================
+ Hits           418       466   +48     

Impacted Files	Coverage Δ
apprise_api/api/views.py	100.00% <100.00%> (ø)

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