refactor package data accessing, set up for per-package options transformer functions

[noahlemen]

branched from #50, with some additional refactors (diff)

new approach for #10 after closing #47

currently only includes options transformation for class properties -- will add others here if this approach seems good to move forward with

changes:

• adds getNewPackageName and getNewPackageOptionsTransformer to be used instead of accessing packageData directly:
  • if package data is an array or a string, getNewPackageName returns that
  • however, package data entries can now be objects instead: { name: string, optionsTransformer: options -> options }
  • if package data entry has an optionsTransformer, getNewPackageOptionsTransformer -- defaults to returning an identity function
• further abstracts parsing of config items from refactor upgradeConfig.js #50 with a ConfigItem class
• updates snapshots for class properties

[noahlemen]

not tied to this method name -- maybe toRawConfigItem would be better? (based on the naming i've used elsewhere... that i'm also not totally satisfied with)