Let target's context win

Given the following config:
```js
preprocess: {
    options: {
        context: {
            ENV: 'DEV'
        }
    },
    dev: {},
    production: {
        options: {
            context: {
                ENV: 'PROD'
            }
        },
    }
}
```
You would expect `ENV` to be `"PROD"` when running the `production` target. This is not the case in 3.0.1.

This change flips the order of `origOptions.context` and `options.context` to let the latter win.