Allow to use extended Hugo Version

[DirtyF]

- Do you want to request a feature or report a bug?

It's a feature 🙏 , not a bug.

- What is the current behavior?

From version 0.43 Hugo comes in two flavours, extended version allows native Sass compilation.
Netlify currently does not support extended version:

2:28:30 PM: ERROR 2018/07/10 12:28:30 error: failed to transform resource: failed to transform “scss/main.scss” (text/x-scss): this feature is not available in your current Hugo version

- What is the expected behavior?

Either download the extended binary by default from now on or allow to specify something like 0.43/extended in netlify.toml.

_extended is added to the release path before the version number:

https://github.com/gohugoio/hugo/releases/download/v0.43/hugo_0.43_Linux-64bit.tar.gz
https://github.com/gohugoio/hugo/releases/download/v0.43/hugo_extended_0.43_Linux-64bit.tar.gz

[jensamunch]

i tried manually including binary in the repo - as a quick-fix - doesn't work:

6:20:18 PM: Executing user command: ./hugo --config paceup.com.toml,config.toml
6:20:18 PM: ./hugo: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.20' not found (required by ./hugo)
6:20:18 PM: ./hugo: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by ./hugo)

[fool]

drat. We apparently ALSO need to install the dependencies for CGO bindings: netlify/build-image#183 so that this will even work.

[calavera]

Fixed by #17