Support lead variables

[pauldix]

Should work the same as #142 but for leading data points. select value, lead(value, 1) from some_series

[jvshahid]

The same result can be achieved using lag. The difference between lead and lag is the timestamp of the point. with lead() the timestamp is that of the older point where with lag() it's the timestamp of the newer point.

[beckettsean]

As mentioned in my post to the mailing list we are experimenting with simplifying our open GitHub Issues. This feature request has been rolled into an aggregate issue for all function requests, so that we can close this issue until we are ready to work on it.

You may continue to make comments here. Closing the issue does not mean we are rejecting this idea.