Move entries to options.entries, provide parsers option

[webketje]

Similar to @metalsmith/sass and @metalsmith/js-bundle use the entries option.
Similar to @metalsmith/markdown's render option, provide a parse option

metalsmith.use(
  metadata({
    entries: {
      nav: 'src/nav.yml',
      featured: 'src/featured.yml'
    },
    parse: {
      '.yml': customYamlParser,
      '.csv': csvParse
    }
  })
)