Safety Valve Override

- Support 'allow.empty.filter', defaulting to 'yes', but if 'no' disallows the
  combination of a write command and an empty filter.  Ordinarily this is just
  a warning that requires confirmation (thanks to Lee Lieske).

src/commands/CmdShow.cpp

@@ -69,6 +69,7 @@ int CmdShow::execute (std::string& output)
   std::string recognized =
     " abbreviation.minimum"
     " active.indicator"
+    " allow.empty.filter"
     " avoidlastcolumn"
     " bulk"
     " burndown.bias"