Allow processing global metadata with the keys option

[webketje]

Currently the keys option is an array that will check all file keys.
There are use cases where it is desirable to be able to format global metadata (for example when fetching rich text translations from a third party provider). A flexible implementation could change the keys option to accept both an array and an object. When an array is specified it is remapped to keys.files for backwards-compatibility, and when an object is defined both are allowed. In a future version the current keys: Array signature can be removed

EG.

ms.use(markdown({
  wildcard: true,
  keys: {
    global: ['labels.*'],
    files: ['faq.*',- 'description']
  },
  globalRefs: './global-refs.md'
})