Added flag to signal if resulting function value should be stringified or not

[enver]

As discussed in #101 added flag to signal if resulting function value should be stringified or not. By default it is set to true to keep backward compatibility.

With this fix you can properly format numbers with comma as a decimal point. e.g.

fields: [
...
{
  value: function(row, field, data) {
    return row['price'].toLocaleString();
  },
  label: 'Price'
  default: 0,
  stringify: false
},
...          
]

and resulting CSV should look like this:

Quantity;Price;Unit
1;22,3;"kg"

instead of:

Quantity;Price;Unit
1;"22,3";"kg"

[coveralls]

Coverage decreased (-0.7%) to 99.329% when pulling ad606e8 on enver:master into e706c25 on zemirco:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 2bf617d on enver:master into e706c25 on zemirco:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 2bf617d on enver:master into e706c25 on zemirco:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 2bf617d on enver:master into e706c25 on zemirco:master.

[knownasilya]

Looks good :)

[knownasilya]

Released as v3.11.0