As briefly discussed in #1425, source the `common/functions` file via an
absolute path, rather than determining the relative path by using
`dirname`. 3rd-party plugins should follow suit and use the new
`$PLUGIN_PATH` convention too.
- 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
Closes#284Closes#683Closes#763
Refs #503
Refs #678