Add option to disable stats

[wilhelmklopp]

As discussed in office hours yesterday, the stats plugin isn't very scalable after a certain point.

You can now set the environment variable DISABLE_STATS, and the stats plugin won't load

cc @JasonEtco @bkeepers

[codecov]

Codecov Report

Merging #392 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
+ Coverage   95.03%   95.07%   +0.03%     
==========================================
  Files          18       18              
  Lines         262      264       +2     
  Branches       30       31       +1     
==========================================
+ Hits          249      251       +2     
  Misses         11       11              
  Partials        2        2

Impacted Files	Coverage Δ
lib/plugins/stats.js	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b71b2a0...aa3e639. Read the comment docs.

[bkeepers]

cc #380, which this is a temporary workaround for.