Local Hook.env

[joshgillies]

Currently Hook.env is global, as stated by:

"Here you can set key / value pairs that will be available in all your hooks as Hook.env" -- https://hook.io/env

It would be nice to have an env per hook. Especially in cases where where across multiple hooks an env key would be something generic like env.apiKey.

The current implementing could be expanded to simply expose an interface via /user/hook/admin that enables the setting of per hook env vars while behind the scenes manages a mapping between env.locaArg (local) and env.hook.localArg (global).

Could be bundled in part with the work happening over at: #108

[Marak]

Sounds good.

Shouldn't be too hard to add.