Added extra arguments to onvif platform config

[karlkar]

Description:

FFMPEG platform allows to set additional parameters in the config. ONVIF platform didn't allow that until this CL. There were always arguments provided by author of onvif platform plugin '-q:v 2'. Somebody may want to use another arguments, like '-pred 1 -q:v 10'.

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#4425

Example entry for configuration.yaml (if applicable):

camera:
  - platform: onvif
    host: 192.168.1.10
    extra_arguments: -pred 1 -q:v 2

Checklist:

• The code change is tested and works locally.

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

• Documentation added/updated in home-assistant.github.io

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

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[fabaff]

Looks good to me 🐦