Make the cobra command factory function customizable

[szkiba]

Make the cobra command factory function ( ̇cmd.New`) customizable with optional event handlers.

To make the k6exec cobra command reusable, it is necessary to provide the ability to use cobra command global event handlers (for example, PersistentPreRunE). If a given handler is also used by k6exec, it must be wrapped in k6exec.

In addition, the k6exec.Command function should also be customizable by passing an optional handler function to cobra command factory function.