Make coverage and snapshot Jest options overridable in package.json

Created by: ryansully

Solves #1455 (closed).

These options are now configurable:

• collectCoverageFrom
• coverageReporters
• coverageThreshold
• snapshotSerializers

Verified by creating a new project using this code and testing with example package.json configuration:

"jest": {
  "collectCoverageFrom": [
    "src/**/*.{js,jsx}",
    "!<rootDir>/node_modules/",
    "!<rootDir>/path/to/dir/"
  ]
}