Add ability to set custom cache paths in middleware to support multiple domains

[michaellindahl]

Closes #99.

Until this is merged it can be using in composer using:

"require": {
    "silber/page-cache": "dev-master"
},
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/michaellindahl/page-cache"
        }
    ],

[JosephSilber]

Can you please add to the documentation how you would handle that in the Apache/nginx configuration? This package works hand-in-hand with that, so it's necessary to spell that out.