Write numbers as numbers

[protobi]

Updated mocha test/example.js as well.

[mattwilsn]

#49

I opened this as an issue.

Looking forward to this update

[dcrystalj]

@protobi
Do you know how to add formula format as well like =(A1+ A2)?

[mattwilsn]

@dcrystalj you need to to do a manual calculations

sheet.set(col, row, 1 + 4)

I do not think there is a way to do any calculations on cells that are already filled

[dcrystalj]

@mattwilsn I need to have formula so others can make some easy changes in excel later. I saw it is possible in exceljs with value type Excel.ValueType.Formula. For me it is quite hard to change whole lib. That is the reason why I am asking for help here

[dcrystalj]

why not merging this PR after so many commits ?