Parse Phoenix-like Params Into Filter Query

[rcdilorenzo]

Currently, it is required to have a complete data structure in order to construct a query from a client. However, this is impractical when trying to do a GET request with filter type parameters. Using the validation and existing features of the library, it would be very helpful to be able to parse parameters such as the following:

/endpoint?completed_date_at_or_after=2016-03-10&title_contains=blah&status=completed

into something like

%Filtrex{type: "all", conditions: [
  %Filtrex.Condition.Date{column: "completed_date", comparator: "at or after", value: "2016-03-10"},
  %Filtrex.Condition.Text{column: "status", comparator: "equals", value: "completed"},
  %Filtrex.Condition.Text{column: "title", comparator: "contains", value: "blah"}
]}

This parsing would allow use of the complex ecto queries from url parameters and yet allow for customizability.

The additional verbiage of "or_after" or "contains" would be parsed specifically by the condition type and would still have the whitelist configuration for allowed keys on the model that the existing parse call has.