:idle-priority keyword to change the running order of idle functions

Lower-priority idle functions are run first. Idle functions with no
specified priority default to 5 and all functions with the same priority
are run in the order in which they are evaluated, meaning the behaviour
is backwards compatible.

Updated documentation as well.
Merge pull request #98 from ffevotte/idle-priority

Add a :idle-priority keyword to change the running order of idle functions