Add simple query for data elements in a Treant.

[dotsdl]

Although the keys for all data elements currently display by default using, e.g. Treant.data, it would be useful to be able to get a listing of data keys that match a query. This could be as simple as making some kind of Data.isin method that takes a string as input and outputs all keys that have that string present.

Originally placed in datreant/datreant#1

[dotsdl]

Perhaps use a name like globfilter to give a similar API to other elements in datreant.