Elaborate median and percentile info in verbose output

I'm slightly ashamed to admit that I don't know what this means:

```
Median mobile site image weight
17 kB
```

nor this:

```
+4.59 MB compared to sites in the 25th percentile
```

The `--verbose` output could perhaps elaborate that part? I could submit a PR, I just don't know what this output means and would appreciate pointers. 😅 I read #8, but it didn't help me fully understand.