Revisit `valueBox()` implementation

[gadenbuie]

The valueBox() implementation in flexdashboard is a little unusual when compared with similar implementations in other packages. We might want to revisit how value boxes are created in flexdashboard or move toward recommending an external implementation.

Here are a few related issues:

• valueBox arguments inconsistent  #283
• Comments below valueboxes do not display as expected #253
• Value Box Display Blank Page #111
• Ion icons don't work in valueBox() with runtime:shiny #309
• Theming flexdashboard: Base font size does not effect valuebox's caption text size and slider inputs #368
• Value-box totally different if Row or Column added #393
• Custom text for valueBox() #408
• Specifying font color for valueBox(). #409