Add a way to separate actions and filters in different (markdown) files

[remcotolsma]

bin/wp-documentor parse wordpress --format=markdown --type=actions --relative=tests/docs --memory-limit=-1 --output=tests/docs/wordpress-actions.md

bin/wp-documentor parse wordpress --format=markdown --type=filters --relative=tests/docs --memory-limit=-1 --output=tests/docs/wordpress-filters.md