Make feature sequences the default

[sgillies]

Current default is feature collections. Would be a major breaking change, but it seems to me that feature sequences are more flexible than collections are and scale better. What say you @perrygeo @geowurster?

[geowurster]

This is in no way a perfect test, but I did a quick GitHub search and couldn't find any projects using cligj that are not directly related to us, so while this is a major breaking change it appears to be safe.

Most instances are from people adding the rasterio and/or fiona source code to their git repo.

cligj -user:mapbox -user:perrygeo -user:toblerity