Add the `hoa test:clean` command.
Support plural options and `--namespaces`.

Plural options:
  * `--file` becomes `--files`,
  * `--directory` becomes `--directories`,
  * `--library` becomes `--libraries`.

Add the `-n`/`--namespaces` option to select a test suite based on a
namespace.
Update usage.
Plural options and add `-d`/`--dry-run`.