Add raw mode to Term aggregation

[dubrowgn]

See also #364

However, the accepted solution to change the way the field is indexed in elastic search seems silly, as the aggregation I'm looking to do is literally already done on the discover screen. If Discover can aggregate a field as a raw string, why can't I do the same thing in Visualize?

Thanks!

[rashidkpc]

Discover is not doing an aggregation, its just listing the document values. This is an elasticsearch limitation, we have no control over it.