RFC: carve out Bramble's core into an importable package

[nmaquet]

Bramble is currently a monolithic and opinionated gateway, which makes supporting all use cases quite difficult. Separating out Bramble's core gateway logic into its own importable package would allow more use cases to be implemented by the community.

[zquintana]

Is there any movement on this? I personally think this is a really good idea too. Might make it easier to implement some of the other features too like #99 .