Add data statistics script for MDN 2019 key result goal

[Elchi3]

This adds a script that gathers data for the MDN 2019 key result goal as presented in #3555

Inspired by https://github.com/atopal/browser-compat-data-dashboard

[ddbeck]

A couple minor notes here, apart from the null matching question. Thank you for doing this, @Elchi3!

[ddbeck]

A couple more questions. Thank you, Florian!

[Elchi3]

@foolip What do you think about this now?

[foolip]

Looks great! One suggestions for improvement, but nothing that would change the behavior.

[foolip]

Note that this will also traverse into strings and arrays, with key being 0, 1, 2 and so on. processData will get called with 1-char strings as arguments and do nothing. This is pretty harmless, just something I was bitten by when writing very similar code with different conditions in processData.

[Elchi3]

I think this is good enough for now. Thanks everyone! Let's land it.

[foolip]

Here are the stats at commit fb736a5:

browser	real values	true values	null values
total	55.20%	22.21%	22.60%
chrome	65.82%	23.95%	10.23%
chrome android	50.25%	32.02%	17.73%
edge	53.23%	20.25%	26.52%
firefox	73.64%	13.10%	13.27%
ie	60.59%	14.77%	24.64%
safari	49.16%	19.07%	31.77%
safari ios	41.11%	20.38%	38.52%
webview android	47.77%	34.14%	18.09%

I did some tweaking of the script to see which are was the most incomplete, and it looks like it's api.*, and APIs on Safari on iOS is the browser+bucket category the most incomplete. CSS data isn't too bad overall.