Use projection option instead of reprojectTo

The `projection` option for vector file sources currently has no good meaning.  The `reprojectTo` option is used to set the projection for cached features.  This change makes it so the projection option is used for this purpose.  The getProjection method (not currently exported) can then be used to get the cached feature projection.

This also removes the default of 'EPSG:3857' for vector file sources.  If a projection is not configured, features will not be transformed.

Fixes #1631.