Add support for Organization Unit field

[tdabasinskas]

Adding support for a new metric ssl_cert_subject_organization_units, which lists the organizations units specified via OU field on the certificate.

While I'm on that, I did the following cleanup as well:

• Add .gitignore file.
• Fixed the example target, as it was not working without https:// prefix.
• Remove redundant commas before and after label values (let me know if there was an actual reason these were added).

[tdabasinskas]

Thanks for a quick response, @ribbybibby.

I've updated ous variable name as requested.

As for these commas, thanks for the link - I wasn't aware about this pattern before. I've dropped that commit so these extra commas are preserved.

[ribbybibby]

Thanks @tdabasinskas.