Added support for other formatters, such as JSON

Hi There,

We are using your plugin and we needed to generate the cucumber JSON report so we could use some other reporting mechanisms such as cucumber-reports.

To achieve this, i made a small change to how the CucumberFormatter is constructed such that it allows you to specify in CucumberConfig.groovy the following:

```formatter = ["json:target/cucumber.json"]```

Allowing you to provide other formatters to use.  They are then chained together (through the DelegatingFormatter) so all of them get executed.

Its been very useful for us.