shell: new `fail_with` and `succeed_with` matchers.

* specs/shell_spec.yaml: Specify behaviour of new matchers.
* lib/specl/shell.lua (matchers.fail_with)
(matchers.succeed_with): Implement new behaviours.
* NEWS: Update.

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

Conflicts:
	lib/specl/shell.lua
	specs/shell_spec.yaml