Add a way to manually add sponsors

[nzakas]

GitHub currently doesn’t integrate invoiced sponsors into their recurring payments systems, so these sponsors end up as one-offs and our script doesn’t pull their info.

I contacted GitHub and they said a fix is still months away for the API, so in the meantime we need to hardcode those sponsors to give them the recognition they deserve.

I’m thinking of creating a _data/custom-sponsers.JSON file that will become the base data that our script adds to when it pulls from GitHub.

Steps:

• [ ] Create _data/custom-sponsors.json file
• [ ] Update _tools/fetch-sponsors.js to read _data/custom-sponsors.JSON and output combined data

[nzakas]

As it turns out, these new sponsors are represented in our current system but I missed them. So, no need to do this.