Support parse config code from main files

[g4s8]

Parse main func (or any other func) with env config parse call, e.g.

env.ParseWithOption(&c, env.Options{Prefix: "FOO_"})

adding to this, if setting a prefix with env.ParseWithOption(&c, env.Options{Prefix: "FOO_"}), that prefix is also ignored.

Originally posted by @caarlos0 in #54