add range support for bleve based search

[joeblew99]

Please provide for the range terms of bleve

numeric range and date range as explained here: http://www.blevesearch.com/docs/Result-Faceting/

@tzdybal

[tzdybal]

@joeblew99, For numeric and date time attributes bleve is not used by dgraph.

[joeblew99]

@tzdybal ok thanks. I will kick the tires on master to see how bleve goes for me.
is there any example code yet that uses it ?

[tzdybal]

Discussion moved to discuss.dgraph.io.