Add support for --config

[kevmoo]

For both build and serve

[kevmoo]

Chatting w/ @natebosch about this. Need to be careful to not expose functionality we're not 98% sure will be around in 6+ months

[jakemac53]

Given that we now have --release I don't think this is necessary, unless we get requests for it from users.

[kevmoo]

Will close as not planned...