Support bold background attribute

termbox sends:
- fg: AttributeBold          -> SGR:Bold
- fg: AttributeUnderline -> SGR:Underline
- fg: AttributeReverse   -> SGR: Reverse
- bg: AttributeBold        -> SGR: Blink

But there is no style option for bg: AttributeBold in peco.
This pull reqeust enables to set bold background attribute.
これ、元のファイルのフォーマットのまま、ってのが多分いけてないですね

```
`"blink"` for blinking background (note: internally this actually sets the `termbox.AttrBold` attribute for various reasons see #165)
```

くらいにしておくと後で混乱しないかな。