Add optional network setting

[jasonmead]

No description provided.

[bpholt]

This PR addresses #4

[bpholt]

I'd like to see some scripted-based integration tests demonstrating that the container created with the new network option is actually attached to that network.

It would be a good idea to add a negative test proving that it's not added unless it's asked for, but I see that we don't have a scripted test demonstrating the defaults for a container created by the plugin, so I'm ok with not adding the negative test for now. (If you want to add it, go for it.)

[bpholt]

Is --network mutually exclusive with --link or can both exist for the same container? If they're mutually exclusive, I wonder if we should express that with the type system somehow?

Update: they are not mutually exclusive.

[bpholt]

We should update the README with the new option before this is merged too.