After update to vscode 1.4 preview should pass Window Title for the new Tab

[Hunter21007]

in registerCommand like this:

        var label = 'nomnoml.diagram';
        return vscode.commands.executeCommand('vscode.previewHtml', previewUri, vscode.ViewColumn.Two, label)
            .then(function (success) {  }, function (reason) {
                vscode.window.showErrorMessage(reason);
            });