Option to return source only for ThreeJS etc

see #11

Works like this:

```js
var myShader = glslify({
    vertex: './vertex.glsl',
    fragment: './fragment.glsl',
    sourceOnly: true
});

//optionally do something with our uniforms/attribs
console.log( myShader.uniforms );

//setup custom ThreeJS material...
var mat = new THREE.ShaderMaterial({
    uniforms: { ... threejs uniforms ... },
    vertexShader: myShader.vertex,
    fragmentShader: myShader.fragment
});
```