Support for grapple.nvim

[serhez]

It would be great to support grapple.nvim (alternative to Harpoon). This could be done by considering the grapple tags instead of open buffers. Additionally, it could be better to just allow the user to provide a function which returns a list of buffers in cybu's config (the default would be nvim's internal buffer list), so that grapple, harpoon and any other similar plugins can be integrated by the user in the config, instead of having specific integrations with each possible plugin.