wishlist: /proc location parameter

It would be great if you could tell sys.json to read proc from another directory such as /proc-host. Then you should be able to run docker like so:

```
docker run -it -p 5374:5374 -v /proc:/proc-host:ro go-sysjson
```

And the docker image can run:

```
sysjson -proc=/proc-host
```
