bucharest-gold / opossum / 327

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feat: Add health check function to a circuit breaker (#76)

## Add `circuit.healthCheck(func[, interval])`

Exposes a way for users to provide their own health check function. The provided function
should return a promise. If the promise is rejected, the circuit will open and the `health-check-failed` event will be emitted.

## Usage
```js
const circuit = opossum(myFunction);
circuit.healthCheck(_ => {
    // check the status of the remote service
    if (remoteServiceDown) {
       return Promise.reject('Cannot connect to remote service');
    }
    return Promise.resolve();
  }, 10000); // check interval - default is 5000

circuit.on('health-check-failed', (err) => {
  // send a text message to the poor sysadmin
  sendErrorText(err);
});
```

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