Remove tfoot if not needed

[toddheslin]

Thanks for this library! I was thinking of building it myself but you've gotten off to a good start.

I'd like to contribute, but just have a small request for now so I'll paste it as an issue.

In your buildFooter function, if no totals are used, there is a blank <tfoot></tfoot> added. This doesn't look great on tables with borders as it appears to be a blank row. Add this patch to the buildFooter function to return an empty string:

        if (content.length > 2) {
          return '<tfoot><tr>' + content.join('') + '</tr></tfoot>';
        } else {
          return ''
        }

[toddheslin]

Just realised that my logic here was based on the current table I was testing. Submitted a pull request :)