customTriggerProc and corrected behavior if triggerPageThreshold == 0

### Option customTriggerProc ###
customTriggerProc allows to specify a function which customizes trigger rendering (like customLoaderProc for loader).
Use case: When IAS is applied to the table, it helps to move the trigger
out of the table
### Corrected behavior if triggerPageThreshold == 0 ###
If triggerPageThreshold == 0, there is no point in binding to the "scroll" event, and, if the next data exists, trigger should be shown immediately.
Right now it's not shown from the start, user is to scroll page just to make it appear.