add cache option

[tamtakoe]

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.

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