Change lib/tag-resources.js:tagResources to be a class called ResourceTagger

[jhford]

Since this function has a lot of context which is annoying to have to pass around for each invocation, let's instead have a class called ResourceTagger which we can pass into the constructors of other classes.

This class should have the skeleton:

class ResourceTagger {
  constructor({runaws, ec2, keyPrefix}) { }

  tag({region, workerType, ids}) { }
}