[FR] Aggregate multiple benchmark output files

**Is your feature request related to a problem? Please describe.**

I run a benchmark and store the results in JSON format. I'd like to have an option to aggregate the repetitions of the same benchmark from multiple JSON files.

**Describe the solution you'd like**

```
compare.py merge a.json b.json
```

**Describe alternatives you've considered**

I can write such a script myself. But maybe such solution already exist and I just can't find it.