Split format function and null formats

[andreyea]

This pull request allows specials characters (; \ * ) to be inside double quotes in format definition. For example, "***";"*_".

If a format string is defined as #,###;; (return formatted value for positives and return empty string for negatives and zeroes), it will treat all null definitions as a single space. This is a valid format in Excel.

This also covers issue SheetJS/sheetjs#2569