'configure', 'build add', 'build targets' now work

- plugin now shows lines containing general Waf commands above the
  actual build-target list:
    [Waf v.....      ]
    configure
    build all
    build targets
    [Build-Targets   ]
    ...

- when the user presses <CR>/<Return>/<Enter> on one of these waf commands:
    configure        => execute cmd ":mak configure"
    build all        => execute cmd ":mak build"
    build targets    => execute cmd ":mak --targets=<selected targets>"

- when showing build-command in preview window, now "mak" is substituded
  by content of vim option &makeprg, so commands now look e.g. like
    "./waf --targets=..."