Handle multiple auth results

[moorereason]

This PR attempts to resolve #45. From the main commit message:

The DMARC spec allows for zero or more DKIM results and one or more SPF results. Prior to this commit, we only allowed for a single result for each type. Update the dmarc.AuthResults struct to use a slice for each type.

For reference, see the definition of AuthResultType in Appendix C of RFC 7489.

This change in the data model required updates to the Go templates. The two HTML templates are almost exactly the same, so pull the common body out into a separate const variable to ease maintenance.

Additional comments to help evaluate the code changes:

• Modified the existing test xml files to add at least one test with multiple DKIM results.
• On the HTML report, separate multiple results with a <br/> tag.
• The text template got a little complicated. I'm not sure I'm happy with the control structures, but it should be fairly simple to understand. When dealing with additional DKIM results, print an additional line with only the DKIM result:

• 111.22.111.33 | 1 | none | pass | fail | abcde.com | pass | abcde.com | pass | | | | | | sendgrid.info | pass | | |

[moorereason]

I forgot to mention that this PR is a breaking change for the JSON output since the data model changes. If anyone has automatic processing of the JSON output, they'll need to update their code.

[tierpod]

Great! Thank you

breaking change for the JSON output

I'll mention it in UPGRADING.md for the next release :+1: