Add GetOffset method on Table

[sachaos]

Firstly, Thank you for your great job!

I propose GetOffset method for Table struct.

Context

I'm developping application which switching between force scrolling mode and row selectable mode dynamically. (like a less +F command option)
I was trying to get current visible rows & cols number and set the top of the visible rows as selected row, by func (t *Table) Select(row, column int) *Table.
But the rowOffset, columnOffset field was private, and no getter method for these.
So I implemented the getter method.

Use case

	table.SetDoneFunc(func(key tcell.Key) {
		...

		if key == tcell.KeyEnter {
			table.SetSelectable(true, false)
			table.Select(table.GetOffset()) // Set the top of visible rows as selected row.
		}

		if key == tcell.KeyEscape {
			table.SetSelectable(false, false)
		}

		...							
	})

[rivo]

Yes, this is a good idea, thank you.