Suppress OnRoute transfers

[MoKob]

Splitting out from https://github.com/mapbox/directions-transit/issues/66:

When a trip contains an OnRoute transfer, we currently would issue a new leg for this trip, since the line changes. In reality, we stay on the same line, though. We should suppress these types of transfers in our output, as it exposes internals right now, which should be hidden from the user.

Tasks:

• [ ] add a test fixture that contains an actual loop in the middle
• [ ] implement suppression of on-route transfers