* Delivers binary name, paths and download URL
  from lib/extensions.js.

* Allows user to set binary name as environment
  variable with `SASS_BINARY_NAME`.
  * Note: this will supersede default name.
* Allows user to set binary name as in package
  file `"nodeSassConfig": { "binary-name": "" }`.
  * Note: this will supersede default name and
    `SASS_BINARY_NAME` environment variable.
* Allows user to set binary name as parameter to
  invoke any node-sass script with
  `--binary-name` flag.
  * Note: this will supersede default name, name
    defined in package `nodeSassConfig` as well
    as the `SASS_BINARY_NAME` environment
    variable.

* This binary name will be used to construct the:
  * Binary path.
  * Binary download URL.
  * Upload URL.

* Allows user to set binary path as environment
  variable with `SASS_BINARY_PATH`.
  * Note: this will supersede default path.
* Allows user to set binary name as in package
  file `"nodeSassConfig": { "binary-path": "" }`.
  * Note: this will supersede default path and
    `SASS_BINARY_PATH` environment variable.
* Allows user to set binary path as parameter to
  invoke any node-sass script with
  `--binary-path` flag.
  * Note: this will supersede default path, path
    as well as the `SASS_BINARY_PATH` environment
    variable.

* This binary path will be used when:
  * Requiring node-sass package.
  * Downloading binary.
  * Uploading binary.

* Wraps all extensions in `process.sass`
  namespace.

Issue URL: #712.
PR URL: #743.