docs(README): add missing syntax highlight annotations (#57)

Author: Josh Goldberg

README.md

@@ -31,7 +31,7 @@ require("expose-loader?libraryName!./file.js");
 
 For example, let's say you want to expose jQuery as a global called `$`:
 
-```
+```js
 require("expose-loader?$!jquery");
 ```
 
@@ -40,15 +40,15 @@ Thus, `window.$` is then available in the browser console.
 Alternately, you can set this in your config file:
 
 webpack v1 usage
-```
+```js
 module: {
   loaders: [
     { test: require.resolve("jquery"), loader: "expose-loader?$" }
   ]
 }
 ```
 webpack v2 usage
-```
+```js
 module: {
   rules: [{
           test: require.resolve('jquery'),
@@ -64,15 +64,15 @@ Let's say you also want to expose it as `window.jQuery` in addition to `window.$
 For multiple expose you can use `!` in loader string:
 
 webpack v1 usage
-```
+```js
 module: {
   loaders: [
     { test: require.resolve("jquery"), loader: "expose-loader?$!expose-loader?jQuery" },
   ]
 }
 ```
 webpack v2 usage
-```
+```js
 module: {
   rules: [{
           test: require.resolve('jquery'),