Add -expiretiles option

[olt]

Write files with coordinates of tiles that need to be updated.

• [x] fixes some bugs identified by #117
• [x] expire BBOX for small polygons (similar to osm2pgsql)
• [x] expire outline for large polygons
• [ ] review definition of "large polygon" (it's 500 tiles right now)
• [x] long lines are rasterized (Bresenham)
• [x] unit tests
• [x] integration tests
• [x] documentation

[stirringhalo]

Instead of hardcoding the 500 tiles to expire for large polygons, could that be passed as an arg to make more flexible? Thanks!