Customize location of vendor dir

It would be nice if I could specify the directory where goop installs requirements. For instance, I'd want to use a .goenv directory instead - I use .virtualenv for python, and .rbenv for ruby.

I'm not sure what this would look like - it looks like you might parse the Goopfile after you create the NewGoop instance, so adding it to the Goopfile is out. Perhaps an environment variable? GOOP_VENDOR_DIR?