Add image options like marked renderer

[fengkx]

Add image options to support prepend root / post asset. and image lazy load attribute.

Usage

# site config file
...
markdown:
  image:
    prependRoot: true # whether perpend root in img src
    postAsset: true # whether perpend root using post asset
    lazyload: true # whether add loading=lazy attribute
...

[fengkx]

@curbengh