add cache option

I can generate some content manually. F.e. I use config.js for local work, but for production I use other config. It would be good to put some files contents in the cache. Moreover, the use of cache (`cache: true`) optimizes inlining files repeatedly.

``` js
.pipe(inline({
    cache: {
        'config.js': 'my content'
    }
}))
```
