Fallback to Table's Converters to format cell values

Currently, each implementation (Excel, CSV and PDF) decide how to handle each cell value.
If the table has columns with converters, they are ignored and a toString is used.

This change introduces the following behavior:
If the exporter implementation knows how to handle the value class (for example, if it's a Date, ExcelExporter can format the cell as date value), nothing changes and it should still use the original value.
If the implementation has no specific way to handle that class, and the table has converters set for that column, the exporter implementation has the option of using this "presentation value" as converted by the Converter.