Fire an event when nzbget download completes

[chriscla]

Description:

Raise the nzbget_downloaded_nzb event when a download completes. Event includes nzb name, category, and status.

Pull request with documentation for home-assistant.io: home-assistant/home-assistant.io#10838

Example automation:

- id: '1571277970615'
  alias: Toggle when TV download completes
  description: ''
  trigger:
  - event_data:
      category: tv
    event_type: nzbget_downloaded_nzb
    platform: event
  condition: []
  action:
  - data:
      entity_id: input_boolean.nzbget_test
    service: input_boolean.toggle

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the [development checklist][dev-checklist]

If user exposed functionality or configuration variables are added/changed:

• [X Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• [No Change ] [The manifest file][manifest-docs] has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• [No Change] New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• [No Change] Untested files have been added to .coveragerc.

If the code does not interact with devices:

• [N/A] Tests have been added to verify that the new code works.

[balloob]

Looks good! This is ok to merge once you have addressed the lint issues

[chriscla]

@balloob -- I can't figure out why CheckFormat is failing. I have re-formatted manually and verified by running Black with the same options as in the pre-commit hook. Plus the commit worked. Sorry to bother you with this but any suggestions? Thanks!

Output:
(venv):~/code/home-assistant$ black homeassistant/components/nzbget/init.py
All done! ✨ 🍰 ✨
1 file left unchanged.