scripts: add Welch's T test

[uetchy]

Add a Python script that consumes the JSON output of hyperfine and calculates the probability of welch's t test.

Can edit PR freely.

Fixes #217

[sharkdp]

Very cool, I love it!

I made a couple of small changes. The output looks like this:

▶ python welch_ttest.py almost-the-same.json 
Command 1: sleep 0.1000000
Command 2: sleep 0.1000001

t = 0.2, p = 0.842

The two benchmarks are almost the same (p >= 0.05).

▶ python welch_ttest.py slightly-different.json 
Command 1: sleep 0.100
Command 2: sleep 0.101

t = -4.33, p = 6.75e-05

There is a difference between the two benchmarks (p < 0.05).