Add Generic OAuth Provider (thomseddon#138)

Add OIDC docs + examples

Update go1.12 -> go1.13 + update dependencies + mod tidy

Fix backwards compat on "domain" config + remove "domains" config

Fixes thomseddon#48

Add releases info to README

Fix go-flags dep + formatting

pre-release logging + docs improvements and fixes

Update README + examples for v2

Import f1ba9b5

Add more v2 tests + fixes + improve legacy config parsing