Delegate snippet-type CLI options to the snippet classes.

The CLI option description and examples are now delegated to the snippet classes.

```
    -I, --snippet-type TYPE          Use different snippet type (Default: regexp). Available types:
                                     legacy : Snippets without parentheses e.g. Given /^missing step$/
                                     percent: Snippets with percent regexp e.g. Given %r{^missing step$}
                                     regexp : Snippets with parentheses    e.g. Given(/^missing step$/)
```

I've ordered the snippet-types alphabetically to make it more predictable (I think #each_pair orders differently in 1.8 compared to 1.9 and 2.0).