Add date format to tooltip for "Date to Timestamp"

[zoeyfyi]

Original issue: zoeyfyi/Boop-GTK#307

It would be useful to have the format of the date accepted by the "Date to Timestamp" script in the description/tooltip. As of ES5 Date.parse defaults to ISO 8081, so prehaps the tooltip could read "Converts dates (YYYY-MM-DDTHH:mm:ss) to Unix timestamp" or prehaps "Converts ISO 8081 dates to Unix timestamp"

Info on EMCA script date time format can be found here.