Update homepage: announce Sculpin 3 and new features

[beryllium]

Sculpin 3 is not quite released yet (it's at RC1 right now and has been for a while), but part of the rollout for v3 will involve updating the website to discuss it.

Should talk about new features:

• init command for creating a VERY basic skeleton (much more basic than the sculpin-blog-skeleton repo)
• content:create command for creating new content types & boilerplate
• New --source-dir and --output-dir options for generating a site

... and about the new PHP 7.2 minimum version requirement.