Builds the site into _site/.
Usage
mix cherry.build [--source DIR] [--out DIR] [--drafts] [--future] [--json]--source— site root containingcherry.exs(default: cwd)--out— output directory (default:SOURCE/_site)--drafts— include posts markeddraft: true--future— include posts dated after today
With --json, emits the standard envelope with page and asset counts.