Allow an existing tilelive instance to be injected

...via Vector = require("tilelive-vector")(tilelive)

While removing the local `tilelive` dependency seems like a positive long-term change, this implementation doesn't impact existing usage.

`Backend` still requires `tilelive` directly, so this is only a partial fix.

(This addresses the `npm link` use-case where different copies of `tilelive` are loaded and the registries differ.)