feat(allow-symbols): Better symbol selection

[bengreenier]

BREAKING CHANGE: allow flag moved to allow-file

This change adds --allow-symbol and moves allow to --allow-file for finer control of symbol selection. We also add some docs, and move our generate-dogfood command to use this feature for time_t and it's backing types.

[bengreenier]

need to rebase before merge.
need to ensure BREAKING_CHANGE ends up in the squash commit body