Bug #1022

- Fixed bug #1022, where dependencies were note released when a blocking task
  was completed (thanks to Arkady Grudzinsky).
- The Task object now caches ::is_blocked and ::is_blocking Booleans that are
  determined on pending.data load.
- Simplified and sped up color rule processing using cached values, reducing
  the number of map lookups, and removed loop invariants when the rules are
  not defined.
- Simplified urgency calculations given the cached values for blocked/blocking.
- On load, pending.data is scanned for accurate blocked/blocking status
  determination.
- Obsoleted and removed complex single-task dependency calculations.
- Sped up 'nag' processing by using cached values..
- Modified the 'show' command to consider color.blocking to be valid.
- Added default config value for color.blocking, and included it in the
  precedence list ahead of blocked, as it is more important.
- Updated taskrc.5 man page to include the new color.blocking rule, and its
  place in the rule precedence.

src/Task.h

@@ -59,6 +59,9 @@ public:
   float urgency_value;
   bool recalc_urgency;
 
+  bool is_blocked;
+  bool is_blocking;
+
   // Series of helper functions.
   static status textToStatus (const std::string&);
   static std::string statusToText (status);