TW-311: Estimated completion in burndown.daily shows impossible results

- Thanks to Michele Santullo.
- Although TW-311 references impossible results, there are two problems.
  First there is the algorithm that determines estimateѕ completion, and
  second there is the reference to wait dates. The algorithm has been replaced
  by something better, but wait dates have nothing to do with estimation.
- The 'burndown.bias' configuration setting is removed.
- The estimated completion is based on the net completion rate since the
  high water mark of total pending tasks, measured on a daily basis, regardless
  of the type of chart produced.
- Vim syntax updated.
- Docs updated.

src/Config.cpp

@@ -95,7 +95,6 @@ std::string Config::_defaults =
   "recurrence.indicator=R                         # What to show as a task recurrence indicator\n"
   "recurrence.limit=1                             # Number of future recurring pending tasks\n"
   "undo.style=side                                # Undo style - can be 'side', or 'diff'\n"
-  "burndown.bias=0.666                            # Weighted mean bias toward recent data\n"
   "regex=yes                                      # Assume all search/filter strings are regexes\n"
   "xterm.title=no                                 # Sets xterm title for some commands\n"
   "expressions=infix                              # Prefer infix over postfix expressions\n"