Add the ability to exclude steps

@NathanGRomano - Interested in something like this? It seems a little overly simple, but it came from an actual use case for me. I'd like to use stairs for flow control in a commander cli app, where certain steps only get added to the flow when a certain option is passed. Like:

```javascript
stairs().step('write', options.write, function() {
  // . . . .
}).step('add', options.add, function() {
  // . . . .
}).step('commit', options.commit, function() {
  // . . . .
});
```

Of course, I could do that in the function itself with this.skip, but if I know in advance that the step won't be necessary, why include it in the flow.

Thoughts?