Cannot find module 'koa' when running first program
After installing koa with `npm install koa -g` in the windows command line, I tried to run my first koa program.
```
const Koa = require('koa');
const app = new Koa();
app.use(ctx => {ctx.body = 'hello koa';});
app.listen(3000);
```

However, the error goes like this:
> internal/modules/cjs/loader.js:960
>   throw err;
>   ^
> Error: Cannot find module 'koa'

I installed the 'events' module before and it worked when requiring 'events' module. Don't know why it doesn't work with 'koa' module and how to fix the problem.