[Proposal] Ignore cells for testing

[longstone]

I use simple-excel to diff generated excels, this is a continuous process. It would be nice if there is a ignore on defined cells.

Example

java.lang.AssertionError: Comparing DemoExcelFile
Expected: entire workbook to be equal
     but: cell at "B4" contained <"2013-12-05 12:40:49.309"> expected <"2013-12-06 12:41:29.424">,
[..]

[tobyweston]

Also asked for in #9.

Think it's a good idea but don't think I'll have any time soon to get to it. PRs welcome.