Argument to handle "unspecified" fields

[mgirlich]

Given an object to tibblify and a specification for this there might be some fields that are not specified. Now, this could be because:

• you don't care about these additional fields
• an optional field has changed the name or there are new fields
• some fields simply vary

The specification should help to clarify what's the case (a bit similar to readr::cols() vs. readr::cols_only()). Therefore, the spec should get these arguments:

• unspecified_fields = c("inform", "warn", "error", "ignore", "collect")
  • "ignore" implies that you know there are (or might be) other fields but you deliberately ignore them
  • "error" implies that you don't expect other fields, e.g. useful when getting data from an API and storing it
  • "inform"/"warn" not sure if both are needed
• unspecified_fields_to is a string specifying the column name where unspecified fields should end up in (similar to tib_multi() How to handle fields whose specification cannot be guessed? #38).
• known_fields to specify the keys of other fields you know exist but you decide to ignore

Things to consider

• a known field is always optional
• a field can be in known_fields and have a specification
• allow known_fields to be a list so that they can be used for nested keys as well

[mattwarkentin]

Is there no way currently to silence the message emitted by tibblify::tibblify() when there are unspecified columns other than wrapping it with suppressMessages()?