Add purchase_url to free bikes and stations

Applications consuming GBFS would like to show markers for the available bikes and stations on the map, and when the user taps on these show a deep link that they can follow to rent that particular bike (for free bikes) or a bike at that particular station (for station-based systems).

Currently, to implement this feature rental URLs need to be hard-coded in the application for each bike/share vendor/deployment.

This patch adds the capability to discover these rental_urls directly from the GBFS feed for each free bike and station.

gbfs.md

@@ -189,6 +189,7 @@ stations          | Yes       | Array that contains one object per station in th
 - post_code       | Optional  | Postal code where station is located
 - rental_methods  | Optional  | Array of enumerables containing the payment methods accepted at this station. <br />Current valid values (in CAPS) are:<br /><ul><li>KEY _(i.e. operator issued bike key / fob / card)_</li> <li>CREDITCARD</li> <li>PAYPASS</li> <li>APPLEPAY</li> <li>APPLEPAY</li> <li>ANDROIDPAY</li> <li>TRANSITCARD</li> <li>ACCOUNTNUMBER</li> <li>PHONE</li> </ul> This list is intended to be as comprehensive at the time of publication as possible but is subject to change, as defined in [File Requirements](#file-requirements) above
 - capacity        | Optional  | Number of total docking points installed at this station, both available and unavailable
+- purchase_url    | Optional  | A fully qualified URL where a user can rent a bike at this station_id.  This URL should be a deep link specific to this station_id, and should not be a general rental page that includes information for more than one station.
 
 ### station_status.json
 
@@ -216,6 +217,7 @@ bikes             | Yes       | Array that contains one object per bike that is
 - lon             | Yes       | Longitude of the bike. The field value must be a valid WGS 84 latitude. See: http://en.wikipedia.org/wiki/World_Geodetic_System
 - is_reserved     | Yes       | 1/0 value - is the bike currently reserved for someone else
 - is_disabled     | Yes       | 1/0 value - is the bike currently disabled (broken)
+- purchase_url    | Optional  | A fully qualified URL where a user can rent this bike_id.  This URL should be a deep link specific to this bike_id, and should not be a general rental page that includes information for more than one bike.
 
 ### system_hours.json
 Describes the system hours of operation. A JSON array of hours defined as follows: