Add and enable Flow type checking.

.babelrc

@@ -1,5 +1,9 @@
 {
+    "plugins": [
+        "transform-flow-strip-types"
+    ],
     "presets": [
-    "es2015"
+        "es2015",
+        "stage-0"
     ]
 }

.flowconfig

@@ -0,0 +1,9 @@
+[ignore]
+.*/node_modules/
+.*/lib/
+
+[include]
+
+[libs]
+
+[options]

package.json

@@ -29,11 +29,14 @@
     "babel-cli": "^6.5.1",
     "babel-core": "^6.5.1",
     "babel-istanbul": "^0.6.0",
+    "babel-plugin-transform-flow-strip-types": "^6.5.0",
     "babel-preset-es2015": "^6.5.0",
+    "babel-preset-stage-0": "^6.5.0",
     "babel-register": "^6.5.1",
     "codecov": "^1.0.1",
     "cross-env": "^1.0.7",
     "deep-diff": "^0.3.3",
+    "flow-bin": "^0.22.0",
     "gaze": "^0.5.2",
     "jasmine": "^2.3.2",
     "mongodb-runner": "^3.1.15",