Hermit strategy

[alecthomas]

We have a github-releases strategy that we'll need to reuse somehow for Hermit, but we'll also need a more generalised hermit strategy that can fetch any endpoint. Could this be generalised so it's not Hermit specific? eg. it's something like this:

// Caches request for github.com/.../releases/...
github-releases {}

// Caches any request to ${CACHEW_URL}/upstream/<host>/<path>
upstream {
  redirect = {
    // Redirect requests for github.com to the `github-releases` endpoint.
    "github.com": "${CACHEW_URL}/github.com"
  }
}