Allow user to provide content/sourcemaps

[Rich-Harris]

This would save having to hit the filesystem if everything is already in memory. Would be particularly useful for e.g. gobblejs/gobble#22

Suggested API:

sorcery.load( 'some/generated/code.min.js', {
  content: {
    'some/minified/code.min.js': '...',
    'some/transpiled/code.js': '...',
    'some/original/code.js': '...'
  },
  sourcemaps: {
    'some/minified/code.min.js': {...},
    'some/transpiled/code.js': {...}
  }
}).then( chain => {
  /* ... */
});

[Rich-Harris]

Released in 0.5.0