added --version switch, fixes #303

It does seem a bit slow though. Usually you expect `myProgram --version` to be quite fast, but there is a noticeable delay before the version is shown with jsdoc. A bit of experimentation suggests to me that the delay is introduced by the large bank of `require('various/modules');` statements at the start of `main()`, so I guess it can't really be helped (unless one only requires the bare minimum modules needed for the `--version` switch and requires the rest after that. It would make the code not as easy to follow though).