[WIP] Add fields param to limit fields returned by Airtable

[mhsnook]

This is a very quick/minimal PR to address #272. I haven't tested it with this plugin or Airtable.js but I thought it would serve as a good addition to the conversation over on issue 272.

But I did curl test the Airtable API and confirmed that yes they are expecting an array of fields and no you cannot send them a string or a blank array. (e.g. this request worked as expected: https://api.airtable.com/v0/{base-id-redacted}/Campaigns?maxRecords=1&view=public_posts_for_internet&fields%5B%5D=Name&fields%5B%5D=Region)

[jbolda]

Note: This is waiting on conversations from #272 to complete.

[jbolda]

We have moved over to https://github.com/gatsby-uc/plugins to maintain the plugin. It seems that this PR and discussion has stalled. If we still need to address this, we can open the discussion there. Closing this.