Proxy support

* Basically bootstraps off of the ENV_DIR support last worked on
* Allows end users to specify proxy aware custom buildpacks
* Example: https://github.com/MakoLabs/heroku-buildpack-nodejs/blob/master/bin/compile (looks for https_proxy)

So my original comment was referring to the redundant use of `${var:-''}`, the second and third args were not being passed in, so it may as well be:

```bash
local blacklist_regex='^(PATH|GIT_DIR|CPATH|CPPATH|LD_PRELOAD|LIBRARY_PATH)$'
```