Add options for customizing preview window title and position.

[rmagatti]

Adds file path and title position to the floating window title if enabled

[README.md]

• Add options for stacking floating preview windows
• Add option for setting the preview window title as the filename [lua/goto-preview/lib.lua]
• Add the file path to the floating window title if enabled
• Add the title position to the floating window if enabled [lua/goto-preview.lua]
• Add a preview_window_title configuration option
• Reformat get_config function in lsp_configs

Closes #92