Allow list values to be raw strings. Fixes #289

[fiznool]

This fixes two separate issues, both to do with setting a custom template for a tcomb list type.

The result is that you can use the following schema & options:

const schema = {
  photos: tComb.list(tComb.String)
};

const options = {
  fields: {
    photos: {
      // Set a custom template for the list
      template: MultiPhotoTemplate
    }
  }
};

with the following template:

class MultiPhotoTemplate extends React.Component {
  render() {
    const { value } = this.props;
    // Here, 'value' is an array of strings, so do what you need to do with them.
    // To update the form field, call `this.props.onChange(newValue)`, where
    // `newValue` is an array of strings.
  }
}

[compojoom]

@alvaromb - I was getting an error with the following code
const tPosition = t.struct({
latitude: latitude,
longitude: longitude
})
const model = t.struct({
polygon: t.maybe(t.list(tPosition)),
})

with the changes from this pull request it works as expected and my polygon is being properly validated (or at least doesn't trigger an error)