Allow IFilterOptions.condition to be set to a function, as the documentation describes.

ui-grid/ui-grid-tests.ts

@@ -41,6 +41,9 @@ columnDef.filter = {
     selectOptions: [{value: 4, label: 'test'}],
     disableCancelButton: false
 };
+columnDef.filter.condition = (searchTerm:string, cellValue:any, row:uiGrid.IGridRow, column:uiGrid.IGridColumn):boolean => {
+    return true;
+};
 columnDef.filterCellFiltered = false;
 columnDef.filterHeaderTemplate = '<div blah="test"></div>';
 columnDef.filters = [columnDef.filter];

ui-grid/ui-grid.d.ts

@@ -3844,7 +3844,7 @@ declare module uiGrid {
          * or you can supply a custom filter function that gets passed the
          * following arguments: [searchTerm, cellValue, row, column].
          */
-        condition?: number;
+        condition?: number | ((searchTerm:string, cellValue:any, row:IGridRow, column:IGridColumn) => boolean);
         /**
          * If set, the filter field will be pre-populated with this value
          */