Using mount point in configuration

[jsilvestre]

Last expression version mainstreams the use of mount point. So we can now use app.use '/a/random/point', americano.static path.resolve(.... This is especially relevant when you want to serve assets from various sub folders (like /public/) without doing dirty trick (like this https://github.com/cozy/cozy-files/blob/master/server/config.coffee#L7-L12).

We can support this feature with the current configuration format, but that will lead to more weird stuff that we'll have to document (and we must be as backward compatible as possible). It's kinda related to #23 in my mind.