add support for connecting flights/stopovers

[amn41]

Users sometimes tell carbon bot that they have a connection / two flights / a layover. We currently don't support that, and it would be great to.

Some scenarios:

• user first specifies first leg of their flight (London to Singapore), then later mentions second leg (Singapore to Sydney)
• user says "I'm flying from London to Sydney via Singapore"

Carbon bot should be able to provide estimates for these multi-leg flights, potentially with a breakdown of the emissions from each leg

[koernerfelicia]

Noting here that I removed the above support for stopovers when upgrading to 2.0 as part of this PR for simplicity's sake.