Vendor support

[dwlnetnl]

In Go 1.6 the GO15VENDOREXPERIMENT environment variable is internally set to 1. But the code checks explicitly for the variable.

Aside from the detection, there is also the watching of the vendor directory tree. I think we can ignore this directory if the vendoring experiment is enabled. This reduces the open file count which is a good thing for macOS.

Any thoughts? No problem to prepare a PR.

[nathany]

Thanks for bringing this up. GO15VENDOREXPERIMENT is going away completely in the imminent release of Go 1.7, which means Looper shouldn't be looking for that environment variable at all.

Given that fsnotify is still using kqueue on macOS, it may make sense to stop watching vendor/ for now. Though once we have FSEvents, I think it might be fine to watch vendor for changes but still exclude it when running all the tests.

[nathany]

I need to give Looper some ❤️ sometime. There are a few things I want to improve, not just this.

Don't worry about a PR. Thanks!