make API for requesting extract generation in planned on-demand generation

[matkoniecz]

Sometime in the next 1-2 months, we are planning a switch to a core set of 100-200 major metro areas, and an on-demand process for others.

https://github.com/mapzen/metroextractor-cities/issues/453#issuecomment-214468974

That makes perfect sense - but it would be nice to include some simple REST API allowing to

• request generation of extract
• get date of last update of extract

[matkoniecz]

For context - I sometimes reload several databases based on mapzen extracts and I automated this task. I would prefer to keep this automated, without necessity to request reloading on some AJAX based www page.

[matkoniecz]

Or maybe entire API thing is unnecessary (maybe it is doable to handle "on demand" transparently for user).