Poor Performance for Large Tables

[dashingdove]

When inserting large tables, the following code performs terribly:

This actually seems to be the fault of the docx package, as the cell function is not very efficient. I have raised this issue over there too. However, if you instead call Table._cells once before the loop and then refer to that array inside the loop, then it improves the performance drastically.

[dashingdove]

Fix: #58