Option to add empty cells

[kukicola]

Hello again! I had one more issue with this gem. It's a pretty strange excel formatting behavior - if a cell with an overflowing text is next to the cell which does not exist in file it allows the text to overflow. Example: Adding blank cell after this one fixes the issue: On my fork I dealt with it by just removing condition to skip empty cells: https://github.com/paladinsoftware/xlsxtream/pull/2/files

Maybe it would be good to introduce an option like empty_cells: which is false by default. I can create a PR if you think that it can be useful.

[sandstrom]

This is expected behavior in Excel and not something that xlsxtream should try to work around imao. I'd vote for closing this isssue.