Add field to DataField and DataConcept for excluding from search index

Fix #172.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Ok, thinking about the defaults more, the flag can be set based on the type in the [heuristic function](https://github.com/cbmi/avocado/blob/master/avocado/core/utils.py#L63-L75) which is used by the sync command.