Support for basic auth

[EnTaroJankov]

Hello,

I want support added for authenticating with username and password, so that people who don't have permissions to create API keys can still make use of this program in restricted Grafana instances.

Thanks!

[EnTaroJankov]

I realized that the underlying Grafana Client, grafana-tools https://github.com/grafana-tools/sdk/blob/888ef9d18611f1503efe233ad507fca8cc8fcf53/rest-request.go#L61, already supports basic auth by passing in username:password. It would be nice to mention this in the documentation and to rename the apikey field to apiKeyOrBasicAuth since it is already supported as is.