Set options per target

I needed the ability for targets to set or override those of the options object. For example:

```
phpunit: {
    options: {
        bin: 'vendor/bin/phpunit',
        configuration: 'phpunit.xml'
    },
    local: {
        coverage: true
    },
    ci: {
        coverageClover: 'build/coverage.xml'
    }
}
```

I wanted to submit a quick PR just to see whether I was going about this correct (is there a current way to do this?) and whether you would accept a PR fixing this.

Thanks for the library!