Add formatters

[juanjoDiaz]

Add formatters to allow the user to customize how different Javascript types are converted into text for CSV.

This would fix the 2 use cases explained in #420 and will provide a better solution for #444.
Also, I'm planning to add a second argument to formatters indicating if it's a header or not) which will fix the use case explained in #195.

[azozello]

@juanjoDiaz Hi I would like to help with this feature, is it still actual ?

[juanjoDiaz]

Hi @azozello this feature is already implemented and merged to master but not merge because there are other pending things for v6.