First pass at official user-env-compile plugin

- Uses ENV and APP/ENV files
- Supports old BUILD_ENV files (which are likely in wide-use)
- Allows user's to override globals with app-specific configuration

TODO:

- [ ] Migrate `$DOKKU_ROOT/BUILD_ENV` to `$DOKKU_ROOT/ENV` if the former exists and the latter does not
- [ ] Drop `BUILD_ENV` support in favor of just `ENV`
- [ ] Allow configuring of global `ENV` file
- [ ] Add default `ENV` with `CURL_TIMEOUT` and `CURL_CONNECT_TIMEOUT`

Closes #284
Closes #683
Closes #763

Refs #503
Refs #678