classModifiers = (base, classKey, context) => {
  const modifiers = []
  switch (classKey) {
    case 'label':
      modifiers.push(`${base}--${context.labelPosition}`)
      break
    case 'element':
      const type = context.classification === 'group' ? 'group' : context.type
      modifiers.push(`${base}--${type}`)
      // @todo DEPRECATED! This should be removed in a future version:
      if (type === 'group') {
        modifiers.push('formulate-input-group')
      }
      break
    case 'help':
      modifiers.push(`${base}--${context.helpPosition}`)
      break
    case 'form':
      if (context.name) {
        modifiers.push(`${base}--${context.name}`)
      }
  }
  return modifiers
}
Given a class key and a modifier, produce any additional classes.
@param {string} classKey
@param {Object} context