Support multiple job files in config.yml

[bentzimortaboola]

Hi

I would like to separate job files per context and config.xml to have array of job files to load, like in https://github.com/free/sql_exporter
this will give the flexibility to add/remove/change files containing the queries and not having one huge file with all queries inside, also if we have different intervals to run jobs it will be much easier that way.

[dewey]

Willing to accept a PR for that if it's implemented in a backwards compatible way.