Omega: Missing support for newer weighting schemes

[Vivek Pal]

> Hi, Vivek — there isn't any particular reason that I'm aware of. It's
> probably worth pointing (in the omegascript documentation) to the part of
> the getting started guide which talks about the different weighting schemes

If there isn't any reason then I'd like to send in a patch adding support for
those weighting schemes in weight.cc and I agree omegascript documentation
should also be updated to point to the weighting scheme section in the getting
started guide ($set command to be specific if I'm not wrong) which I'll try to
incorporate in the same PR.

Additionally, omegascript documentation will also require updating to reflect
support for newer weighting schemes in $set command.

> It doesn't cover Dir+, but it does talk about BM25+ and PL2+.

Actually, Dir+ is mentioned under LMWeight weighting scheme section in the
last paragraph, although in brief.

Thanks,
Vivek