Support package_root_url from empack package data

[trungleduc]

Reference

Closes #68

Code changes

pyjs will use the package root URL defined in the package data generated by empack to construct the download URL. If this root URL is not defined, it will fall back to the default package_tarballs_root_url as usual.

Backwards-incompatible changes

N/A

[trungleduc]

cc @DerThorsten @martinRenou

[DerThorsten]

lets call this optional extra field in in the package not package_root_url but just url and use this directly as the URL if present. Ie instead of concatenating the package_root_url with pkg.filename we directly use url if url is existing in the pkg object

[trungleduc]

lets call this optional extra field in in the package not package_root_url but just url and use this directly as the URL if present. Ie instead of concatenating the package_root_url with pkg.filename we directly use url if url is existing in the pkg object

I agree, will update the PR. On a side note, does the file extension matter? Or I can save my packages to other file extensions than tar.gz?

[DerThorsten]

tar.gz

Do you just want to have a different ending or do you actually want to use a different compression format?
Atm is has to be a gzip'ed tarball.
The actual ending of the file should not matter that much

[trungleduc]

Just ending. I learned with a client that their proxy blocks downloading tar.gz file but not others. They can use this URL config to trick their proxy