Questions about this plugin.

Hi, this is really nice plugin, but I have some questions/feature requests as following:

#### Isn't it better to use command instead of provisioner?

In my opinion, a provisioner should provision something to vagrant instances, so how about to use command instead of provisioner? 

#### Multi vm support

Any plans to support multi vm environment? like as this [gist](https://gist.github.com/riywo/5579491)

#### Spec exclusion support

In case of sharing specs both vagrant and actual server(see following), it is better to have exclusion feature.

```ruby
describe "my-host" do
  it "should be pass both vagrant and production" do
    # brabrabra
  end 
  it "should be pass only production", :production => true do
    # Of cource, we need different way to execute this example like as executing `rake acceptance`  or etc..
  end
end
```

If you have plan or agree these features, I am glad to contribute:)