create-react-context as optional peerDependency

create-react-context is only is needed as a polyfill for React 15 and lower; yet, the inclusion of the library blocks React 17 support. As an "optional" peerDependency, all user needs should be fulfilled while maintaining the spirit that the React libraries (and any needed polyfills) are external to the Router library itself.

package.json

@@ -28,7 +28,13 @@
   },
   "peerDependencies": {
     "react": "^15.0.0 || ^16.0.0 || ^17.0.0",
-    "react-dom": "^15.0.0 || ^16.0.0 || ^17.0.0"
+    "react-dom": "^15.0.0 || ^16.0.0 || ^17.0.0",
+    "create-react-context": "^0.3.0"
+  },
+  "peerDependenciesMeta": {
+    "create-react-context": {
+      "optional": true
+    }
   },
   "devDependencies": {
     "babel-cli": "^6.26.0",
@@ -42,6 +48,7 @@
     "babel-plugin-transform-react-remove-prop-types": "^0.4.13",
     "babel-preset-env": "^1.6.1",
     "babel-preset-react": "^6.24.1",
+    "create-react-context": "^0.3.0",
     "eslint": "^4.18.2",
     "eslint-config-react-app": "^2.1.0",
     "eslint-plugin-flowtype": "^2.34.1",
@@ -80,7 +87,6 @@
     "printWidth": 80
   },
   "dependencies": {
-    "create-react-context": "0.3.0",
     "invariant": "^2.2.3",
     "prop-types": "^15.6.1",
     "react-lifecycles-compat": "^3.0.4"