Add direction keyword for advanced background

[yhatt]

This PR adds the direction keyword for advanced background, vertical and horizontal to control aligning direction of multiple backgrounds.

The default direction is horizontal. By adding vertical keyword, user can stack background images vertically.

This feature is based on request in #137.

NOTE: The mentioned core request is splitting contents to multiple layer. We have not planned to implement it, because we afraid to unexpected layout for theme author.

Regular backgrounds

![bg vertical](https://placekitten.com/1000/600)
![bg](https://placekitten.com/800/700)
![bg](https://placekitten.com/900/900)

horizontal (default)	vertical

with Split background

![bg right vertical](https://placekitten.com/1000/600)
![bg](https://placekitten.com/800/700)
![bg](https://placekitten.com/900/900)

horizontal (default)	vertical

ToDo

• Update docs to explain direction keywords

[yhatt]

CI is blocked by a vulnerability of dependency. We have to update dependencies at first in another PR.