Add command-line component generator

Adds a generator that scaffolds up a complete, but minimal, component using JSX and a propTypes hash.

Run `rails generate react:component --help` for usage notes.

Related issue: #72 
This might be chance to push FB code style for multiline returns :)

```
return (
  <div>
    <div>...</div>
  </div>
);
```

I don't care too much, it just allows things to line up nicely and helps prevent ASI when refactoring happens.