Support for more excel features

Hi, I added support for excel formulas and numbers. 

Any string that starts with '*' is treated as a number and any string that starts with '=' is treated as a formula.

Examples: 
* sheet.set(1, 1, "*27"); 
  * puts 27 in cell A1 but 27 does not get added to sharedStrings.xml.
* sheet.set(1, 1, "=A2 * A4");
  * puts =A2 * A4 in cell A1
* sheet.set(1, 1, "=SUM(A2:C2)");
  * puts =SUM(A2:C2) in cell A1, which sums values of cells A2 to C2
* sheet.set(1, 1, "=SUM(A2:C2)~27");
  * does the same thing but sets the default value of the cell to 27

You can also add excel number formats

Example:
* sheet.numberFormat(1, 1, {format: '_("$"* #,##0.00_);_("$"* \(#,##0.00\);_("$"* "-"??_);_(@_)'});
  * formats cell A1 to start with '$' and end with .00 if there is no decimal already