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 |                  |          |