Added callbacks on error handling. Removed the log.error call on the compressor callback to continue processing.
Added callback handling on log.error() calls. Added unit tests and documentation
Code improvements. Added 2 more tests for bad uglifyJS builds on command line and rollups. Improved the documentation.
Ensure all fatal errors callback and make all callbacks flow back to the initial call to shifter.init() or shifter.add().

In init/add either call back if there's a supplied callback or process.exit(1) (in the case of an error) if there is no callback.
Improvements as per pull request feedback.
Simpler examples for testing fatal syntax errors.
Merge branch 'feature/callback-error-handling' of https://github.com/orionhealth/shifter