Support SASS_PATH environment variable

[kfranqueiro]

SASS_PATH is an environment variable present since Sass 3.2 (see http://sass-lang.com/documentation/file.SASS_CHANGELOG.html#Smaller_Improvements10):

Sass.load_paths is initialized to the value of the SASS_PATH environment variable. This variable should contain a colon-separated list of load paths (semicolon-separated on Windows).

This feature is supported by node-sass.

This may also end up being a useful escape hatch for configuring include paths within opinionated "zero-configuration" frameworks such as Create React App 2.