With --list, allow certain tasks to be pinned on top

[hans-d]

with task --list / task --list-all there is only a --sort option.

Certain tasks should be more prominent than others, showing on top and separated by an empty line from the rest of the tasks.

Eg

> task --list

task: Available tasks for this project:
* important         blah
* some:other        do foo bar baz 

* any:other         xxx

Optionally with some header, eg pinned and other