Allow specifying custom headers in `load_data` function

It would be very beneficial to add a headers option to load_data so that protected resources could be fetched by `load_data.

Use-cases: when accessing protected APIs or workaround an issue with GitHub Markdown API [for instance, you need to render a gfm formatted markdown file (e.g. release notes/issues texts)].

Fixed in templates/load_data: add an optional parameter `headers` by liushuyu · Pull Request #1710 · getzola/zola · GitHub