Allow setting default metadata as part of the load phase

[patrickarlt]

Add a second param to load that allows it to assign default metadata to all pages loaded.

Would turn:

acetate.load('**/*.+(html|md)');
acetate.metadata('**/*', {
  footer_margin: 3,
  layout: 'layouts/_layout:main'
});

Into:

acetate.load('**/*.+(html|md)', {
  footer_margin: 3,
  layout: 'layouts/_layout:main'
});

which means one less loop through all pages to assign metadata in the transform step.

[patrickarlt]

Done!