Default file export results in corrupt data

[bzimmer]

If no file is specified on export the contents of the file and the metadata are written to stdout resulting in a corrupt data file.

As an example:

~ > gravl strava export 5391565932

<snip>

 J�E��84�@"�����;�;�J\9`ˌ{
 "name": "Thank_you_.fit",
 "format": "fit",
 "id": 5391565932
}