Add certificate subject as a tag

[onurguzel]

If there is a certificate chain in input source, all certificates are
returned with the same source tag. Subject of the certificate is added
to use all certs in the chain as an input.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

I think we should call this tag cn and it could be a nice touch to add other fields if set such as o, ou: https://tools.ietf.org/html/rfc2253#section-2.3

[onurguzel]

I think we should call this tag cn

At first, I was using the RDN with Subject.String() (Go 1.10+), and then I modified my commit to use CommonName to be able to build on Go 1.9, leaving tag name subject. Should I rename to common_name or the abbreviation cn?

and it could be a nice touch to add other fields if set such as o, ou

Sounds nice. What do you think about a c (country) tag?

[danielnelson]

I guess using the longer name instead of the abbreviation would be better, especially when considering the one letter abbrevs. I think we could do any/all of these:

• common_name
• locality
• province
• organization
• organization_unit
• country

[onurguzel]

I have added all the tags you requested.

[danielnelson]

Thanks, I'll update the readme and I'm going to add this to 1.8.3 since this is critical for the plugin to work properly.