feat(persistence): pre_save option to call before saving (#22)

* feat(persistence): `pre_save` option to call before saving

This commit adds the `pre_save` option, which is a `fun()|nil` that will
be called (if present) before saving the current session. Use cases
include filtering buffers, executing `User` autocmds, etc.

* docs: `pre_save` option

doc/persistence.nvim.txt

@@ -71,6 +71,7 @@ Persistence comes with the following defaults:
     {
       dir = vim.fn.expand(vim.fn.stdpath("state") .. "/sessions/"), -- directory where session files are saved
       options = { "buffers", "curdir", "tabpages", "winsize" }, -- sessionoptions used for saving
+      pre_save = nil, -- a function to call before saving the session
     }
 <
 

doc/persistence.txt

@@ -71,6 +71,7 @@ Persistence comes with the following defaults:
     {
       dir = vim.fn.expand(vim.fn.stdpath("state") .. "/sessions/"), -- directory where session files are saved
       options = { "buffers", "curdir", "tabpages", "winsize" }, -- sessionoptions used for saving
+      pre_save = nil, -- a function to call before saving the session
     }
 <