Backward relations (one-to-one) (#55)

Support filtering on one-to-one backward relations

Author: mattbretl

__tests__/fixtures/queries/connections-filter.relations.graphql

@@ -1,6 +1,8 @@
 query {
   parent_name_equalTo_parent2: allFilterables(filter: {parentByParentId: {name: {equalTo: "parent2"}}}) { ...filterableConnection }
   forward_name_equalTo_forward2: allFilterables(filter: {forwardByForwardId: {name: {equalTo: "forward2"}}}) { ...filterableConnection }
+  backward_name_equalTo_backward2: allFilterables(filter: {backwardByFilterableId: {name: {equalTo: "backward2"}}}) { ...filterableConnection }
+  #child_name_equalTo_child2: allFilterables(filter: {childrenByFilterableId: {name: {equalTo: "child2"}}}) { ...filterableConnection }
 }
 
 fragment filterableConnection on FilterablesConnection {

__tests__/integration/__snapshots__/queries.test.js.snap

@@ -1859,6 +1859,14 @@ Object {
 exports[`connections-filter.relations.graphql 1`] = `
 Object {
   "data": Object {
+    "backward_name_equalTo_backward2": Object {
+      "nodes": Array [
+        Object {
+          "id": 2,
+          "string": "Test",
+        },
+      ],
+    },
     "forward_name_equalTo_forward2": Object {
       "nodes": Array [
         Object {