#674 add support for fsync's noTime flag

This adds a `notime` flag for setting whether or not `fsync` should update the file times on a sync operation. This defaults to false, which preserves the zero value of the `fsync.Syncer` struct.

This is for #574 Sync file dates in /static. This does not address the /static part specifically, but enables access to the requested setting in that post.

This is also posted as http://discuss.gohugo.io/t/file-dates-in-static/179