Add support for plugins installation

[jsenecal]

This project should allow for plugins to be installed in the deployed docker-image.

The netbox-docker wiki has good documentation on this topic.

[jsenecal]

I've edited the original text to remove the PR submission. Unfortunately I had to make a serious amounts of changes to make this project fully compatible with the way we usually deployed netbox on docker-compose that submitting multiple PRs/Issues is not sensible.

You can always cherrypick from my fork as I will be trying to make these commits easy to work with.