Allow multiple API keys

[Daniel15]

It would be nice to allow multiple API keys to be provided, in which case it should send requests using each of the provided API keys.

My use case is that I want to use the read-only monitor-specific API keys (to reduce the risk in case the key somehow leaks), however these API keys only provide access to a single monitor. If I want to scrape five monitors into Prometheus, that'd need five separate API keys. I guess I could run multiple instances of prometheus-uptimerobot, but having a single instance that performs multiple requests seems like a better idea.