feat: expose option to configure the popup border in plugin opts

[tom-gora]

I added an option to configure the popup border as needed this for style consistency. Quick tweak. Could be improved maybe by getting it to respect global "winborder" but that's a task for tomorrow me.

For example configure:

-- ...
		max_width = nil,
		max_height = nil,
		max_width_window_percentage = 40,
		max_height_window_percentage = 35,
		popup_border = "rounded",
		scale_factor = 1.0,
--...

to get this: (here working with version patched with #367 )