formatters: support loading builtin formatters by basename.

* specs/specl_spec.yaml: New specification for using a custom
formatter by basename on the command line.
* specl/specl.mk (specl_install_edit): Do as the documentation
states, and allow loading of builtin formatters with a simple
`--formatter=tap` command line option.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
docs: document improved custom formatter API.

* README.md (Custom Formatters): Document improved custom formatter
API from François Perrad.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>