Feature request: add possibility to define discoveries consumed by prometheus

[lufik]

I would like to propose the ability to blackbox_exporter to provide also list of scrapes to prometheus.
So the admin of blackbox machine can propose also scrapes and not only modules.

What will be the blackbox.yml module config.

This could be added into blackbox.yml, where files contains only the list under configs (and can be generated).

discovery:
  files:
  - <fileglob>
  configs:
  - module: http_2xx
    targets:
    - <host/url>

What will be the prometheus.yml scrape config.

- job_name: "exported-blackbox-probes"
  http_sd_configs:
  - url: http://prometheus-blackbox-exporter:9115/discovery

I try to create v1 implementation so we can easier discuss about it: #1160