feat(maven): Add support for Maven (#3147)

lib/config/definitions.js

lib/manager/index.js

lib/manager/maven/extract.js

lib/manager/maven/index.js

test/_fixtures/maven/simple.pom.xml

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

test/manager/maven/extract.spec.js

test/manager/maven/index.spec.js

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

website/docs/configuration-options.md