Add retry-ability

Adds [retry-ability](https://docs.cypress.io/guides/core-concepts/retry-ability.htm) to snapshot tests, by adding two new configuration options: ``` retryCount - the number of retries that should be done if the test is failing retryDelay - the amount of time between each retry ``` This is useful, for example, for cases when some tests are failing because the image snapshot is done before some images are loaded due to slow network.