specl: add inclusive `--example` filters.

* specs/specl_spec.yaml: Specify behaviour of --example filters.
* lib/specl/main.lua (optspec): Add --example option, along with
reference documentation for --help output.
(execute): Coerce single example options to 1-element tables.
(compile): Append example filters to each spec-file structure.
* lib/specl/runner.lua (run_examples): Adjust filtering to manage
new --example filters.
* doc/specl.md (Running a Subset of Examples): Document --example.
*: NEWS: Update.

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