add event to customize cleaning of cell values

[agallou]

Exemple of usage :

        $('table.highchart')
          .on('highchartTable.cleanValue', function(value, options) {
               if (options.indexTr == 0) {
                 options.value = options.value.replace(/\$/, '');
               }
          })
      .highchartTable()

List of options passed to the event :

• value (that's the one that must be changed, contains the value already cleaned by HighchartTable)
• rawValue : the value not cleaned by HighchartTable
• td : jQuery objet of the td were the valued is extracted from
• tr : jQuery objet of the tr where is td is readed from
• indexTr : index of the tr (starts at 0)
• indexTd : index of the td (starts at 1)

see #59