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.

test/special.t

@@ -38,6 +38,7 @@ if (open my $fh, '>', 'special.rc')
             "color.alternate=\n",
             "color.tagged=\n",
             "color.pri.H=\n",
+            "color.completed=\n",
             "nag=NAG\n",
             "_forcecolor=1\n";
   close $fh;
@@ -47,6 +48,7 @@ if (open my $fh, '>', 'special.rc')
 # Prove that +nocolor suppresses all color for a task.
 qx{../src/task rc:special.rc add should have no red +nocolor priority:H 2>&1};
 qx{../src/task rc:special.rc add should be red +nonag 2>&1};
+
 my $output = qx{../src/task rc:special.rc ls 2>&1};
 like ($output, qr/\s1\s+H\s+should have no red/,      'no red in first task due to +nocolor');
 like ($output, qr/\033\[31mshould be red\s+\033\[0m/, 'red in second task');