specl: add --fail-fast option.

* lib/specl/main.lua (optspec): Add --fail-fast option.
* lib/specl/runner.lua (run): Stop running if run_examples
returns false.
(run_examples): Stop running examples if an expectation fails,
or run_contexts returns false.
(run_contexts): Propagate a false returned by nested context or
example.
* doc/specl.md (Running a Subset of Examples): Document
--fail-fast.
* NEWS: Update.

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