docker run -i -t  / attach support

Add support for attach (hijack proxy)

It's kinda hard to to test, but it seems to be work fine.

Fix #194
Related to #177


@vincentwoo @avaer and @abronan please take a look.
Maybe this could just be `utils.go`?

`getContainerFromVars` is not entirely proxy related, and we might stuff other things into this file.