Allow per-field overrides of field values with placeholders

[patcon]

Hi! Looking for feedback of whether you'd ever accept something like this. If so, I'd be happy to spiff this up with proper docs within this PR.

Use-case is that I want to add guardrails to ensure that some PII (personally identifiable information) from airtable will not get written to a static government website.

Alternative approaches considered:

• somehow hook into schema generation of gatsby. doesn't seem possible (no hooks to modify third party source plugin schemas)
• extracting PII into a separate table. hurts UX of managing airtable, and hard to ask of nontechnical users, who are confused why information is now spread across a bunch of new tables.

Thanks for considering!

[jbolda]

Per #272, it seems that the best fit is proceeding with something as in the vein of #326. Thanks again for looking into this @patcon!