feat: Add homebrew manager (#3530)

lib/config/definitions.js

lib/manager/homebrew/extract.js

lib/manager/homebrew/index.js

lib/manager/homebrew/update.js

lib/manager/homebrew/util.js

lib/manager/index.js

renovate-schema.json

test/config/__snapshots__/validation.spec.js.snap

test/manager/homebrew/__snapshots__/extract.spec.js.snap

test/manager/homebrew/__snapshots__/update.spec.js.snap

test/manager/homebrew/_fixtures/aalib.rb

test/manager/homebrew/_fixtures/aap.rb

test/manager/homebrew/_fixtures/acmetool.rb

test/manager/homebrew/_fixtures/aide.rb

test/manager/homebrew/_fixtures/ibazel.rb

test/manager/homebrew/extract.spec.js

test/manager/homebrew/update.spec.js

test/manager/homebrew/util.spec.js

...rkers/repository/extract/__snapshots__/index.spec.js.snap

website/docs/configuration-options.md