Add data.updateTag request

Add a data.updateTag request to allow for updating/renaming tags.

The request will be called in the following way:

const args = {
  tag: 'current tag name',
  update: {
    name: 'new tag name'
  }
};
surix.request(requests.data.updateTag, args).then(updatedTag => {
  // do something with updated tag
}).catch((error) => {
  // error occurred
});

You should also update the README to include a section on how to use this request.