Allow Specifying Hooks in Environment Variables

[charlespierce]

With tools like npm and yarn, the configuration parameters can be set using config files (.npmrc) or with environment variables (NPM_CONFIG_REGISTRY). To support one-off calls and specific, sandboxed environments, Volta should allow specifying hooks in environment variables as well.

We'll need to investigate how to specify the type of hook (Template, Prefix, Bin). Alternatively, we could provide a single environment variable that points to a hooks.json file that we load.