refactor: simplify option processing.

* specl/version.lua.in (M.PACKAGE, M.PACKAGE_NAME)
(M.PACKAGE_BUGREPORT, M.VERSION, M.Name, M.program)
(M.COPYRIGHT_STRING, M.COPYRIGHT_NOTICE, M.BUGREPORT_NOTICE):
Remove.
* specl/std.lua (prog.version, prog.usage): Move from here...
* specl/version.lua.in (M.version, M.usage): ...to here, and
simplify.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
refactor: generalize std.lua, move Specl specifics to version.lua.

* specl/std.lua (optarg): Remove.
(process_args): Remove Specl specific option processing, in favor
of tabulated general option processor.
* specl/version.lua.in (M): Tabulate option processing in terms
of handler functions.
(M.opts): Default option settings.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
docs: improve installation instructions.

* README.md (Installation): New section.  Describe installation
options.  Renumber following sections.

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