Deprecate marked options at the root of the options object

[webketje]

It is confusing that metalsmith & marked options must be specified at the same level of the options object.

markdown({
  gfm: true,  // -> marked opt
  keys: ['desc'] // -> metalsmith opt
})

This has the risk of name collision and passes irrelevant options to marked.
Better would be, as in metalsmith-in-place & layouts:

markdown({
  keys: ['desc'], // -> metalsmith opt,
  engineOptions: {
    gfm: true,  // -> marked opt
  }
})

For 1.x both will continue to be supported, but add a deprecation warning.
For 2.x root-level marked options will be removed