[Coverage] add rules for minimum coverage threshold

[Bamieh]

Add rules to the .nycrc file to exit(1) when coverage drops in PRs under a certain threshold, thus failing the CI.

Currently the coverage looks like this:

=============================== Coverage summary ===============================
Statements   : 88.46% ( 69/78 )
Branches     : 66.67% ( 8/12 )
Functions    : 84% ( 21/25 )
Lines        : 88.46% ( 69/78 )
================================================================================
Done in 5.14s.