Add ability to create feed within a group

[brentru]

• Added the ability to create a feed within a group.
• Added test case
• Added usage example

API Endpoint: https://io.adafruit.com/api/docs/#create-feed-in-a-group
Addresses issues #104 and #118

[brentru]

I'm going to merge this in, it passes local and forked unit-tests and will pass on master.

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Unit-test failing looks similar to the previous PR, merging into master will authorize the CI script.

Unittest fails to acknowledge the env secrets on PRs (https://github.com/adafruit/Adafruit_IO_Python/pull/125/checks?check_run_id=1590263116#step:7:8) v.s. when the action runs in master.

I believe it's because "secrets are not passed to workflows that are triggered by a pull request from a fork". We would need to work off a branch for future pull-requests. < @dherrada Could you take a look at this/verify that I'm not missing something here?