Enhancement: Add support HEXO's post asset folders

[estruyf]

    This is a HEXO feature. https://hexo.io/docs/asset-folders defines it better. I have the `_config.yml` parameter `post_asset_folder: true` set.

post_asset_folder: true
marked:
  prependRoot: true
  postAsset: true

Originally posted by @lorezyra in #428 (comment)

[estruyf]

@lorezyra just tested it out, with the settings you provided above, it behaves like our page bundles. In this case, FM needs to implement the knowledge of how to use these HEXO settings in order to adapt its behavior.

[estruyf]

@lorezyra with the post_asset_folder setting set to true are you able to reference images from other folders?

[estruyf]

I've added support for HEXO its image functionality.

When you set the frontMatter.framework.id setting to hexo, you can set the frontMatter.content.publicFolder folder to:

• source/images: If you want to use the default image location of HEXO
• hexo:post_asset_folder: If you want to use the post asset folder functionality

When you initialize a new project, these settings are automatically applied, but when already initialized, you will have to manually add this.