Pass additional arguments to xcodebuild

I found that I can not build my project which contains sub projects since `OBJROOT` and `SYMROOT` are required.

This patch is to allow passing arguments to `xcodebuild` from `frank build` command line as the following:

    $ frank build -- OBJROOT=/path/to/project/Frank/frankified_build SYMROOT=/path/to/project/Frank/frankified_build