Add functionality to obtain particular historic database versions via version number.

[jonesor]

Enable the use of cdb_fetch to obtain particular versions of the database.
This would make it easier for users to replicate analyses if they know the version number of the database use. It would also allow users to compare different versions of the database (e.g. "why is my analysis different from the same one I did 5 years ago?".

I envisage something like this:

x <- cdb_fetch("compadre", version = "6.21.1.0")

Doing this will need the help and support of the database team at compadre-db.org because we need to know exactly how to access old stored versions of the database (i.e. how to build the URL to the datafile).