feat(import_image): replace image embed link with relative path

[curbengh]

Continue #70

Downloads all image attachments and save them according to their respective directories.
Example: http://yourwordpress.com/wp-content/uploads/2020/07/image.jpg => http://yourhexo.com/2020/07/image.jpg.

I'm assuming Wordpress store images like this:

http://yourwordpress.com/wp-content/uploads/2020/07/image.jpg
http://yourwordpress.com/wp-content/uploads/2000/01/image.png
http://yourwordpress.com/wp-content/uploads/2005/08/image.gif

After importing the images, with this PR, the original image embed will be automatically updated:

![caption](http://yourwordpress.com/wp-content/uploads/2005/08/image.gif)

becomes,

![caption](/2005/08/image.gif)

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Not compatible with post_asset_folder: true config yet.

[coveralls]

Coverage increased (+0.4%) to 95.455% when pulling 93468e1 on curbengh:update-embed into ff998bd on hexojs:master.