Engine hooks for file writing

[geoffp]

Addresses #541

Summary of changes:

• Ask the linter to tolerate es6 syntax; all the es6 syntax herein was chosen to be compatible back to Node 4.4
• Factor out writePatternFiles()
• Preserve the current scheme as the default formatter, and provide hooks for engines to be able to implement their own
• Add a hook for engines to be able to add files to the output if they need to

[bmuenzenmeyer]

This looks like a very smart solution. Thanks Geoff!