Integrate Transport for London API for enhanced public transport features

## Problem Statement
- Integrate [TfL's API](https://api.tfl.gov.uk/swagger/ui/index.html?url=/swagger/docs/v1#!/Journey/Journey_JourneyResults) to support more detailed public transport info in the route analysis section.
  - For example, some people may want to avoid the tube.
- Detect routes that only involve London to enable these additional features. 