docs: clarify YAML string quotation rules.

* docs/specl.md (YAML): Not all punctuation forces description
strings to require quotes, only punctuation that is ambiguous to
the YAML parser.
* specs/matchers_spec.yaml, specs/specl_spec.yaml: Remove
unnecessary description quoting.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
formatters: report 'No expectations' instead of 'nan% of expectations'

* specs/formatters_spec.yaml: Move from here...
* specs/custom_formatters_spec.yaml: ...to here.
* specs/formatter/spec_helper.lua: New file. Implement spec
runner helper.
* specs/formatter/report_spec.yaml: Specify correct output from
report formatter.
* specs/specs.mk (specl_SPECS): Add
specs/formatter/report_spec.yaml.
(EXTRA_DIST): Add specs/formatter/spec_helper.lua.
* lib/specl/formatter/report.lua (footer): When there are zero
passing expectations, don't trigger a DIVBYZERO, just report "No
expectations met".
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
formatters: report pending argument strings consistently.

* specs/formatter/spec_helper.lua (run_spec): Accept additional
arguments to SPECL call.
* specs/formatter/report_spec.yaml: Specify output for various
combinations of pending calls in example specs.
* specs/specl_spec.yaml: Loosen specification to allow additional
pending argument display.
* lib/specl/matchers.lua (pending): Always save the argument
as a string, defaulting to "not yet implemented".
* lib/specl/formatter/report_spec.yaml: Display pending argument
strings consistently.
* NEWS: Update.

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