Improve export file name

Could you change export to save file name in better format instead of export-.json I think format like this could be better:

YYYYMMDDHHmmsstypingmind-export.json or YYYY-MM-DDHH-mm-sstypingmind-export.json. This way users will immediately know from the file name when and from what service is this export file.

And the exact timestamp of export generation could be included in JSON itself.