Adding a command to generate the env information for linking

Adding a command to generate the env information for linking for linking aginst a library. This is useful for `keg_only` formula and for generating the correct -I and -L information for any formula including its dependencies.

Basically, if I have an external project that I don't want to put in homebrew but I would like to link against a keg_only formula, this script could help me generate the env variables needed to build my project.