Add support for result paging

[amotl]

To reach beyond the 5000 results limit employed by Grafana on the dashboard search API, grafana-wtf should apply appropriate paging, see also https://grafana.com/docs/http_api/folder_dashboard_search/.

This request originated at #2, thanks @jangaraj.

[amotl]

@jangaraj: As I learned from your reports, you are operating Grafana setups with more than 1000 dashboards but you have not yet crossed the 5000 dashboard mark, right?

Please let me know if this turns out to be true at some time, then we will add appropriate paging mechanisms to the code.

Cheers!

[jangaraj]

Yes, my Grafana instance has only 1.3k dashboards.