Show CPU/memory information before the benchmark-ips output

Right now the output looks like this:

```
Ruby version: 2.3.3
Warming up --------------------------------------
struct 20.863k i/100ms
hash 4.222k i/100ms
Calculating -------------------------------------
struct 225.122k (± 3.9%) i/s - 1.127M in 5.013067s
hash 43.546k (± 4.6%) i/s - 219.544k in 5.053637s
```

I think it'd be better if we saw the CPU/memory information of the environment where it's running. Something like `uname -a` and more?

Anyway, it's just an idea. I'm not sure how useful it would be.