Add field to DataField and DataConcept for excluding from search index

Fix #172.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Yes. Also, I will update the comments to be more clear. 

But, if you and @murphyke see no value for having this at the concept level, I can easily remove it and we can just do this on a per field bases. I have no strong opinion either way.