add pre{major,minor,patch} levels to --help
add pre{major,minor,patch} levels to --help

you49net/Repos,bitovi/node-semver,stealjs/node-semver,megawac/node-semver,thenaughtychild/node-semver,ekmartin/node-semver,stevemao/node-semver,rvagg/sanever,hippich/bower-semver,jasonkuhrt/node-semver,npm/node-semver