shorthand for repeated task prefixes

[notnmeyer]

tsk blah:a :b :c would be the same as, tsk blah:a blah:b blah:c

the prefix should be assumed from the task name on the left. so multiple shorthands could be used,

tsk foo:a :b bar:a :b is the same as tsk foo:a foo:b bar:a bar:b

[notnmeyer]

there's nothing special about the use of colons in task names currently, just how i write them. this change would create the concept of them actually being namespaces or organizational units.