Add support for "H"-prefix in Helsinki (OSM) bus stops

[Antiik91]

This pull request fixes an issue where stop code with H prefix would not work in HSL environment. Because this fix is only meant for HSL, a vertexconnector factory was created in order to provide same functionality as before to other agencies. Now it's also easier to create other custom connectors too.

• [ ] issue: Link to DT-3075 that describes the relevant feature or bug.
• [ ] roadmap: Check the roadmap for this feature or bug. If it is not already on the roadmap, PLC will discuss as part of the review process.
• [ ] tests: Have you added relevant test coverage? Are all the tests passing on the continuous integration service (Travis CI)?
• [ ] formatting: Have you followed the suggested code style?

To be completed by @opentripplanner/plc:

• [ ] reviews and approvals by 2 members, ideally from different organizations
• [ ] before merging: add a bullet point to the changelog file with description and link to the linked issue
• [ ] after merging: update the relevant card on the roadmap