Validators can't contain dashes

```
    jscheme = require('jscheme').new()
    jscheme.add 'min-length-5', (val) ->
      val.length >= 5

    jscheme.add 'contact',
      name: 'string, min-length-5'
```

jscheme parses the validator string `min-length-5` into `min`,
which results in the error `'contact.name: missing validator min',`
