Add context

This commit adds and passes context around to (almost) every function.
This allows cancelling (almost) every function call globally.

pkg/environment/environment.go

@@ -45,7 +45,7 @@ type Environment interface {
 	DialContext(context context.Context, network, address string) (net.Conn, error)
 
 	Executable() (string, error)
-	Exec(io stream.IOStream, workdir string, exe string, argv ...string) int
+	Exec(ctx context.Context, io stream.IOStream, workdir string, exe string, argv ...string) int
 	LookPathAbs(name string) (string, error)
 }