added option to display legend for pie charts

By default pie charts legend are disabled and it was not possible
to activate them (only via datalabels but not with "normal" legends.

Now the data-graph-pie-show-in-legend allows to enable legend for pie
charts.

Author: agallou

jquery.highchartTable.js

@@ -413,7 +413,7 @@
             dataLabels: {
               enabled: true
             },
-            showInLegend: 0,
+            showInLegend: $table.data('graph-pie-show-in-legend') == '1',
             size:         '80%'
           },
           series: {