Allow `.yard/opts` as an alternate for `.yardopts`

Having the *choice* to use `.yard/opts` instead of the `.yardopts` would be a nice feature so projects can be as tidy as possible --that way only one point on reference need be in a project's root, namely the `.yard` directory, instead of both `.yard` and `.yardopts` in cases where both are needed.

I know it must seem a tiny thing, but it means a lot to me. Hope you will accept the patch.