add support for using HUGO_VERSION of `latest`

[nolessafool]

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

feature!

- What is the current behavior?

it is impossible to automatically install the "latest" version of hugo by setting a value for $HUGO_VERSION (you can only track the version and update your settings every time there is a release)

- What is the expected behavior?

Would be nice to give folks some way to install the latest. We did some brainstorming around this at the netlify-internal slack link here

- Please mention your Go version, and operating system version.

N/A