Implement configurable detach key

Implement configurable detach keys (for `attach`, exec`, `run` and
`start`) using the client-side configuration

- Adds a `--detach-keys` flag to `attach`, `exec`, `run` and `start`
  commands.
- Adds a new configuration field (in `~/.docker/config.json`) to
  configure the default escape keys for docker client.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

api/types/client.go

@@ -17,6 +17,7 @@ type ContainerAttachOptions struct {
 	Stdin       bool
 	Stdout      bool
 	Stderr      bool
+	DetachKeys  string
 }
 
 // ContainerCommitOptions holds parameters to commit changes into a container.