Add multi station nearby search and transport type filtering

[guisua]

Solves issues

• #3
• #4

Summary

Nearby searches now support single or multiple returns as list, as well as optional argument transport_types to filter nearby results by specific types.

Changes

Adds methods:

• nearby_multi
• nearby_multi_async

as well as new parameter transport_types for existing methods:

• nearby
• nearby_async

Tests

Corresponding unittests have been added.