Add input tester example (#2068)

#### Is this adding or improving a _feature_ or fixing a _bug_?

Example.

#### What's the new behavior?

Adds a new example that is an input event logger, for more easily seeing which input/keyboard/selection events are firing when editing in a Slate editor.

![image](https://user-images.githubusercontent.com/311752/43937175-ea9231e2-9c10-11e8-8e29-2cd9a24ca78d.png)

#### Have you checked that...?

* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)

package.json

@@ -58,6 +58,7 @@
     "react-hot-loader": "^3.1.3",
     "react-portal": "^4.1.5",
     "react-router-dom": "^4.3.1",
+    "react-values": "^0.3.0",
     "read-metadata": "^1.0.0",
     "rollup": "^0.55.1",
     "rollup-plugin-alias": "^1.4.0",