Add options to customize sidebar collapse icon and location

[asbates]

It would be great if we could change the look and location of the collapse icon for sidebars. More specifically I think it would be good if we could

• change the icon
• change the location of the icon

Maybe the sidebar could have an argument icon similar to many Shiny components. For the location, I think options of 'top', 'middle', and 'bottom' would suit most people's needs.

Example usage:

layout_sidebar(
  sidebar = sidebar("Sidebar"),
  icon = icon("bars"),
  icon_position = "top",
  "Main contents"
)