Convert Config#processors to attr_reader.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Add the ability to process a subset of items.

An item can have a set of cache tags that indicate that a particular process
has already been run on them.  In future runs, only new items can be
processed, and data can then be extracted from all items matching a
particular tag.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>