Add support for a config file

[gdubicki]

It would be cool if this library could read the PuppetDB URL, certificate and key paths and all other connect parameters from a config file.

This would allow easier writing of cli tools using this library as they could all share a common configuration, out of the box.

I propose to use the same path and config syntax as Puppet's own PuppetDB cli as this will provide a single configuration place for users of both the Puppet Enterprise and this library or its derivatives.

If this is something that would get merged then I can provide a PR. :)

[gdubicki]

I would extend the pattern of the PuppetDB cli's configuration file order with reading also ./puppetdb.conf, if it is present.

This would allow the users of multiple PuppetDBs (like testing and production instances) to easily switch between them by cd-ing to the appropriate directory with the right config.

(This is a method that we are already successfully using for the multiple sets of keys for hiera-eyaml.)

We could also consider adding support for providing a configuration file path with an env variable - this is probably another popular pattern for this problem.