Add direction groups for routes in agency routeConfig

[EddyIonescu]

Currently we can define default_directions for routes, where we set 0 to be Inbound and 1 to be Outbound. However, for some transit agencies like the TTC 0 means Eastbound or Northbound, while 1 means Westbound or Southbound. Then there are transit agencies (like Grand River Transit in Waterloo, Ontario) that use Inbound/Outbound for some routes and North/South/East/West for others.
While custom_directions could be used, this would require much less lines as there are about 200 routes, with a rough 50-50 split between east-west and north-south.
Currently:

default_directions:
  '0':
    title_prefix: Eastbound
  '1':
    title_prefix: Westbound

Proposed:

default_directions:
  '0':
    title_prefix: Eastbound
  '1':
    title_prefix: Westbound
custom_default_directions:
  'north-south':
    'routes': [
        '1', '3', '5', '6', '7', '9', '11', ...
    ]
    '0': "Northbound"
    '1': "Southbound"
  'inbound-outbound':
      ...