Update documentation, add option --onFailOnly and update exit code

[piopi]

• ReadMe updated to reflect recent changes
• add -h option to the CLI command
• handle exit code
• add --onFailOnly option to send webhook only when the test fails

Handle exit code with this snippet
.then((results) => { console.log(results) process.exit(results.totalFailed) }) .catch((err) => { console.error(err) process.exit(1) })