Add ability to format category name and series point value.

[tdibacco]

Added two new exporting options for csv.

• exporting.csv.rowCategoryFormatter Formatter callback for the category name of a row. Parameters are point and series. By default it returns null maintaining the current behavior.
• exporting.csv.rowValueFormatter Formatter callback for the value of a row. Parameters are point and series. By default it returns null maintaining the current behavior.