feat: support preset and render options

[curbengh]

Currently when a preset is set, its options can't be overridden and anchor feature cannot be used.

Updated wiki.

• BREAKING CHANGE: config syntax has been updated, old config compatibility is temporarily retained
  • Old config

    markdown: 'commonmark'

  • will be automatically converted to:

    markdown:
    preset: 'commonmark'
    # render, plugins & anchors options are empty

• refactor(test): set default config