Ruler bar style configuration

[antonok-edm]

It'd be really cool if it was possible to customize the style of the ruler bar in streampager's configuration. Changing background/foreground colors would be a good start, perhaps extended terminal attributes like underline/bold/italic could also be supported.

Currently the bar's default style is hardcoded with AnsiColor::Silver as the background and AnsiColor::Black as the foreground. In a lot of dark terminal themes, that silver color is very bright and stands out significantly compared to, say, code with syntax highlighting.

edit: just noticed #47, which mentions color customization as well. It sounds like a config DSL system has already been mostly sorted out in the process of supporting custom keymaps.

Considerations:

• Colors should ideally support traditional indexed colors as well as "true color", perhaps with fallback - i.e. like termwiz::color::ColorAttribute
• Support for extended attributes could be modeled using termwiz::cell::CellAttributes

Alternatively, something as simple as this in the existing TOML config could be fine to begin with:

[ruler.style.normal]
background = "#555555"
foreground = "15"

[ruler.style.information]
# ...