Timestamp command should not add wrapper elements

[aboukirev]

Currently timestamp command wraps date and time in square brackets. According to Org specification, timestamp for closed TODO items uses that wrapper. In various other places timestamp can be used with angle brackets. For example, a range could be represented as <2004-08-23 Mon>--<2004-08-26 Thu>, and appointment could be scheduled at <2006-11-01 Wed 19:15>. There is different semantics attached to different wrappers.

Suggest renaming existing command to inactiveTimestamp and adding a new command agendaTimestamp that will use angle brackets wrapper.