Dependencies - #410

- Completed support for 'task 1 depends:2,-3' to manipulate the
  dependencies.
- Now supports rc.dependency.reminder to indicate when to nag about
  dependency chain violations, defaulting to on.
- Now supports rc.dependency.confirm to require confirmation before
  fixing dependency chains, defaulting to on.
- New source file dependency.cpp which implements a low-level API for
  determining dependency status, and assorted handlers for task state
  changes.
- Adds blocking tasks to the 'next' report.
- Added more dependency unit tests, changed the wording in a couple of
  them and numbered them for easy reference.

src/Config.cpp

@@ -104,6 +104,7 @@ std::string Config::defaults =
   "\n"
   "# Dependency controls\n"
   "dependency.reminder=on                         # Nags on dependency chain violations\n"
+  "dependency.confirmation=on                     # Should dependency chain repair be confirmed?\n"
   "\n"
   "# Urgency Coefficients\n"
   "urgency.next.coefficient=10.0                  # Urgency coefficients for 'next' special tag\n"