Enable placeholder in CharField

[jumbosushi]

Right now CharField data is used to render Template's char field placeholder (passed template.field.label in props to placeholder field):

Would be nice to enable the field to store placeholder info so we can show an example of how the field should be written:

Places to start

1. Allow CharField class on Django to store placeholder info as well around this code
2. Edit the CharField component on React to render this new placeholder in this file to be something like

export default function CharField(props) {
  return (
    <TextInput
      placeholder={props.field.placeholder}
      value={props.data || ''}
      fill={true}
      onChange={e => props.onChange(e.target.value)} />
  )
}