Feature request - Add microsecond as a time unit for interval

The interval datatype can go to microsecond precision though currently 
the smallest unit is seconds.  Microseconds are represented as decimal 
places, eg 5 microseconds is 0.000005 seconds.

To insert microseconds I have to use the following line, ($1*0.000001 || 
' seconds')::interval
Being able to specify microseconds as a unit would avoid hacks like this 
and improve support for sub-second intervals.


David