Improve stemming and corpus search
Fixes #324 (closed) and it should help with purescript-gargantext#633 (closed) .
It piggybacks on !261 (merged) which would require to be merged first.
@anoe This MR refactors a bit the Query
type for corpus search in a way that now we can do normal searches like before, but we now have a ~
operator to use before words to perform the "expert search".
For example in the example that @athomas mentioned in the other issue we can search for ~postpart
and that should yield the results that previously were missing.