shorten config

[ssoroka]

Ideas behind this change:

• reduce duplication by providing standard methods for parsing field types.
• allow reusing fields by not deleting them
• delay unused field errors by instead adding them to a list and removing it if it's later processed
• reduce repetitious error handling by collecting errors on the config
• provide our own toml.Config settings that allows us to capture missingField errors and handle them ourselves.

[ssoroka]

on hold until after 1.15