Label spans - creating label

[herongrove]

As a user, I want to be able to create a label for a span of tokens by
1. clicking and dragging across the tokens, which will highlight the tokens (moved to another issue)
2. entering a label for the span (above the span)
3. pressing Enter/Return to save, or Esc to cancel

The updated span should be reflected in the parsed data. Using the Extended UI example on TAG's homepage, if I were to add a new span on the token by, I could add the label prep. Then I could look at the Parsed data object below the extended example and see that label prep in Object > tokens > 9 > registeredLabels > default > val: "prep".