Support passing more than one GITHUB_TOKEN

[julioz]

At my org, we use several different bot accounts for different workflows. In order to use the exporter now, I can only pass one token at a time (https://github.com/kalgurn/github-rate-limits-prometheus-exporter/blob/master/internal/github_client/github_client.go#L59-L61) which means I have to run N exporters in order to obtain rate limit information about my N tokens.

I believe it would be better, resource-wise, to have a single exporter running, and that exporter iterates through a list of tokens, recording the rate limit metrics for each one of them.