fires on typing text in an input of a column's header
| value | string|number | an entered value |
| colId | string|number | the id of a column |
| filter | string | the type of a filter: "inputFilter" or "selectFilter" |
pivot.grid.events.on("HeaderInput", function(value,colId,filter){
console.log("You've entered "+value+" into the "+colId+" column");
});