feature request: avoid automatic import/export for xlsx files

[sanzoghenzo]

First of all, thanks for this great add-in!

I installed the xlam in excel add-in directory and started using the automatic import/export feature; It works well but every time I open a file it asks to import the src, even if the file is an xlsx that doesn't contain any macro. On save/close, it exports the NamedRanges.csv file.

It would be great to add a check for xlsm and xlam (and xls) extensions.

It also keep asking to import vbaDeveloper.xlam src files, but there's #41 for this already.

[hilkoc]

Thanks for raising this. This is a good improvement. I'd be happy to accept a pull request for this.