new functions for managing existing miniCRAN repo

Major additions:
* add new packages to existing miniCRAN repo
* check for package updates from CRAN-like repo
* add updated packages from CRAN-like repo
* update `twodigitRversion` to handle more types

NOTE: there appears to be a small amount of overlap between the new `add.packages.miniCRAN()` and the existing `addPackage`:
- The new one does more than simply updated the package index, it downloads the files
- Not sure if it's better to reindex the way I've done it or to use your `addPackage` for this purpose

In other functions we use `availPkgs` as the argument name.  For consistency, please review and change if possible.