Enhancement - import command

- Implemented import command
- Implemented Context::clearMessages to remove all accumulated
  messages.  This is needed because parts of the import process are
  recursive and we don't want Context to dump messages for every
  import record on completion.

src/Context.cpp

@@ -572,3 +572,11 @@ void Context::footnote (const std::string& input)
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+void Context::clearMessages ()
+{
+  headers.clear ();
+  messages.clear ();
+  footnotes.clear ();
+}
+
+////////////////////////////////////////////////////////////////////////////////