Expose the `stats` property in each file metadata

A stat(1) call is made for every single file passed to Metalsmith. The only
field exposed in the metadata was mode. This commit exposes the whole stats
object by the stats property so that it is quickly accessible without the need
of a new stat(1) call.

The mode property is left as is so as to avoid compatibility issue, though the
information is now duplicated and is also accessible via stats.mode.

See: https://github.com/segmentio/metalsmith.io/pull/39