Added EnvPlugin.

This adds a convenience plugin that uses `DefinePlugin` to import environment variables. It also warns if you've forgotten to define a variable.

Example:

**app.js**
```javascript
if (process.env.NODE_ENV === 'production') {
  // Do something only in production mode
}
```

**webpack.config.js**
```javascript
var webpack = require('webpack')

module.exports = {
  // ...

  plugins: [
    new webpack.EnvPlugin('NODE_ENV', 'API_URL')
  ]
}
```

This will output the following warning:
```
[EnvPlugin] Warning: API_URL environment variable is undefined.
```

And result in the following javascript output:

```javascript
if (('production') === 'production') {
  // Do something only in production mode
}
```
I actually had it that way in my initial implementation, but that will prevent dead code removal in minification:

```js
if (process.env.NODE_ENV === 'development') {
  // Development debugging code
}
```

Would compile to:

```js
if (({NODE_ENV: 'development'}).NODE_ENV === 'development') {
  // Development debugging code
}
```

Instead of:

```js
if ('development' === 'development') {
  // Development debugging code
}
```