Add support for putting emails and URIs in SAN field

[chiefnoah]

SANS are allowed to contain more than just domain names and IP addresses. This PR adds the ability to specify email addresses and URIs in the Subject Alternate Name field of the generated leaf certificates.

This is of particular interest when using client certificates as authentication mechanisms. An example of this usecase can be found here.

I did a little refactoring while I was in there to update the Go version, ignored the generated artifact from go build, reduce some redundant parsing, and remove the deprecated ioutil package.

[chiefnoah]

rebased this on master and tested locally