Add `merge` option to schema

[howardroark]

The idea here is to add a new "step" which merges the data of json (or other) object files. The first file in each array being the file to write to.

{
  merge: [
    ['package.json', 'PROJECT-package.json']
  ]
}

[jedwards1211]

I will implement this, unless you want to beat me to it

[howardroark]

Go for it!