Add support for .gpx.gz file parsing

[matthewturk]

This adds support for including .gpx.gz files.

I'm using this to include activities from Strava, which (at least in my case) default to being gzipped. The compression from gzip is pretty significant, and I wanted to avoid having to include the activities in uncompressed form.

Because the files have a .gpx.gz extension I had to check for that in the file.path attribute, rather than using file.extension.

[valentine195]

LGTM thanks