Custom Reports - oldest, newest

- Added support for the "report.X.limit" configuration variable, to
  restrict the number of rows a report generates.
- Added support for Table::render (limit) to limit the number of rows
  that are rendered.
- Removed "oldest" and "newest" report code.
- Added "oldest" and "newest" custom report details to Config.cpp
- Updated various documentation.

src/parse.cpp

@@ -130,12 +130,7 @@ static const char* commands[] =
   "history",
   "ghistory",
   "info",
-  "list",
-  "long",
-  "ls",
-  "newest",
   "next",
-  "oldest",
   "overdue",
   "projects",
   "start",