Add `\ruby{}{}`

[Berrysoft]

This is useful when authoring config in Japanese.

We need to adjust the structure of a line. The text of an action should not be a String, but Vec<Lines>:

enum Lines{
    Chars(String),
    Block(String),
}

The chars should be printed one by one, while the block should be printed altogether.

This adjustment will also fix the issue with markdown plugin, because it generates HTML tags.

[Berrysoft]

Two Chars in a Vec is the same as one Chars. We can reduce allocations as a bonus.