Add parseValue to type definitions (#213)

Author: mrodrig

lib/converter.d.ts

@@ -109,6 +109,12 @@ export interface IFullOptions extends ISharedOptions {
    * Specify the keys that should be excluded from the output.
    */
   excludeKeys?: string[];
+
+  /**
+   * Specify how values should be converted into CSV format. This function is provided a single field value at a time and must return a `String`.
+   * Note: Using this option may override other options, including `useDateIso8601Format` and `useLocaleFormat`.
+   */
+  parseValue?: (fieldValue: any) => string;
 }
 
 export function json2csv(data: object[],