Wizard support for Flights related emission queries

Fixes #52 (closed) by adding wizard support for Flights related emission queries and makes the platform more user-friendly.

• Menu support added for Flights
• Slot filling feature added for origin i.e. the source city
• Slot filling feature added for destination i.e. the destination city
• Suggests parameter passengers i.e the number of passengers traveling
• Adds context-management support. Here's a preview of it:-

If the user chooses the option to not provide the number of passengers:-

And if the user opts the option of providing the number of passengers:-