Support non-global coverageThreshold

[deini]

Currently this lib only support reading the global coverageThreshold, it would be nice to support bumping it for others too, eg:

coverageThreshold: {
  global: {
    branches: 100,
    functions: 100,
    lines: 100,
    statements: 100,
  },
  './src/some-dir/': {
    branches: 80,
    functions: 81,
    lines: 90,
    statements: 83,
  },
  './src/some-other-dir/': {
    branches: 100,
    functions: 100,
    lines: 100,
    statements: 100,
  },
},

[thall90]

@rbardini Would you be willing to accept a PR with this functionality?

[rbardini]

@thall90 Absolutely, I'm happy to review any contributions towards this!