Add functions to restore custom buffer bookmark properties

[shipmints]

• New defcustoms 'bufferlo-bookmark-buffer-locals', 'bufferlo-bookmark-buffer-handler-functions'.
• New defuns 'bufferlo-bookmark-map-function-buffer-locals', 'bufferlo-bookmark-buffer-handler-buffer-locals', 'bufferlo-bookmark-map-function-text-scale-mode-amount', 'bufferlo-bookmark-buffer-handler-text-scale-mode-amount'.
• Update documentation.

[shipmints]

I suppose the configuration of these two features could be simplified like we handle ibuffer integration. The hooks need to be installed in pairs so we could automate this. Nobody will want to have only half the hooks installed, right?

[shipmints]

Done.