Fix yarn support

[andrew]

Currently forest doesn't support yarn because yarn always makes https requests to the registry regardless of what options you've set (see yarnpkg/yarn#7393).

The solution is to properly support https requests via the proxy, which requires configuring certificates and such.

[andrew]

Now that #32 is fixed, this should be fixed, but yarn v1 doesn't ignore the domain on the registry field like npm does so it skips loading tarballs from localhost and goes straight back to the registry.

It doesn't cause yarn to fail anymore, but it doesn't cache anything onto ipfs either.

In theory this is fixed in yarn v2, I've not tested that yet, but also I'm not sure if many people are using yarn v2 yet either.