Feature: scheduled date

- Supports 'scheduled' date for tasks, which represent the earliest opportunity
  to work on a task.
- Added unit tests for urgency.
- Implemented color rule.
- Added scheduled to 'info' report.
- Updated assorted documentation.
- Removed comments in default rc because they are never seen.
- Added (broken) 'ready' report, which is like 'next' but only for ready tasks.

src/commands/CmdShow.cpp

@@ -112,6 +112,7 @@ int CmdShow::execute (std::string& output)
     " color.pri.M"
     " color.pri.none"
     " color.recurring"
+    " color.scheduled"
     " color.summary.background"
     " color.summary.bar"
     " color.sync.added"
@@ -185,6 +186,7 @@ int CmdShow::execute (std::string& output)
     " taskd.credentials"
     " undo.style"
     " urgency.active.coefficient"
+    " urgency.scheduled.coefficient"
     " urgency.annotations.coefficient"
     " urgency.blocked.coefficient"
     " urgency.blocking.coefficient"