Provide a way to override server's port

[julioz]

Right now the port is hardcoded to 2112: https://github.com/kalgurn/github-rate-limits-prometheus-exporter/blob/master/internal/prometheus_exporter/server.go#L87

Depending on the setup in which this exporter is installed, that port could already be occupied by another process. The exporter should provide a way to override the port it starts the server on(, and then use 2112 as default if not provided).