Add flatten utility and support for filtering requireAll files

• refactor: remove unused import be1294e
• docs: move flatten method docs out of the helpers block 8b24381
• docs(README): update documentation 908ca14
• feat: add support for defining custom filter for require-all method d609a16
• feat: add flatten utility 938bd1c
• chore: update dependencies 9065c8f

v3.2.1...v3.3.0