Make the "host FS" location configurable

There needs to be a way to pass in the host filesystem location as an option to allow collection from the host machine when operating inside of Docker. This is equivalent to how Metricbeat allows the `-system.hostfs` CLI option.

This must not be set as a global variable as it is in both gosigar and gopsutil.

I'm thinking to have an API like `sysinfo.New(opts types.Option)` and have an option that can be used like `types.HostFS("/hostfs")`. `sysinfo.New` would return an interface that has methods like `Host()`, `Processes()`, `Process()`, and `Self()`.

And I think only the Linux provider would actually honor this option.