Added options substitution in module description.

Function extmod_print_desc now prints module description with
"[[OPTIONS]]" replaced with actual module options usage generated by
extmod_print_opts().
Implemented module extraction from command line arguments.

Now using extmod_extract() function user can extract module from
arguments line. It combines selecting module name and searching for
corresponding module.

Module name is extracted by next rules:
1. if executable name (may be link to executable) is module name - use
that module;
2. otherwise, if first command line argument is module name - use it
and remove it from comman line arguments.

It is recommended to check for correct executable name if this call
returned NULL. Because it would be strange if utility renamed into
nonexisting module name will behave usual way.