Alt-Name Support

The main changes:

• Multi-name flag/option support. Just provide a list of alternate names in { ... }.
  The first match will return.
  Example:
  cmdl({ "-t", "--threshold"}, 128) >> theshold;
  If either t or threshold were specified (dashes are ignored), set threshold to that value.
  Otherwise use the default: 128.

• No need to provide argc to parser.
  You can now write:

  int main(int, char* argv[])
  {
       argh::parser cmdl(argv); // Look Ma! no argc!
       // ....

• Some internal refactoring.