fix: allow border title position change/retaining (#300)

Author: jamestrew

lua/plenary/window/border.lua

@@ -173,12 +173,18 @@ local set_title_highlights = function(bufnr, ranges, hl)
   end
 end
 
-function Border:change_title(new_title)
+function Border:change_title(new_title, pos)
   if self._border_win_options.title == new_title then
     return
   end
 
-  self._border_win_options.title = new_title
+  pos = pos or (self._border_win_options.title[1] and self._border_win_options.title[1].pos)
+  if pos == nil then
+    self._border_win_options.title = new_title
+  else
+    self._border_win_options.title = { { text = new_title, pos = pos } }
+  end
+
   self.contents, self.title_ranges = Border._create_lines(
     self.content_win_id,
     self.content_win_options,