Enhancement: add a parameter to control whether if we want to clear the output window upon start

[pemessier]

It would be nice to control whether if we want to clear the output window upon start:

VSERunOnSave/VSERunOnSave/RunningDocTableEvents.cs

Line 80 in 527227a

OutputString("", true);

For example, if we modify a lot of files (massive search&replace) currently the history only shows the last saved item. If something wrong happened to another file, the external tool output is lost.

Thanks and great work by the way!