Shell resource new function

[alileza]

Is your feature request related to a problem? Please describe.
There is no way we can check exit code from executed command in shell resource

Describe the solution you'd like
Would be nice to add a new function to get latest exit code from executed command

https://github.com/tomatool/tomato/blob/master/handler/shell/shell.go#L10

something like

type Resource interface {
	resource.Resource

	Exec(command string, arguments ...string) error
	ExitCode() (int, error)
	Stdout() (string, error)
	Stderr() (string, error)
}