feat: Add option for user's on exit callback (#144)

## Motivation:

If I close LazyGit after having done some changes (Commit
something/change branch…) I often run into some minor inconsistency with
the UI on screen, that does not get updated to reflect the changes.
In particular the issue I’m having is with the color of the files inside
the [neo-tree](https://github.com/nvim-neo-tree/neo-tree.nvim) view. I
understand this is a really insignificant issue, but I still would like
to fix it.

## Proposed solution:

I have added an option that allow an user of the plugin to add a
callback that is automatically executed after exiting LazyGit, so that I
can put there the necessary code to refresh my UI

lua/lazygit.lua

@@ -35,6 +35,10 @@ local function on_exit(job_id, code, event)
     buffer = -1
     win = -1
   end
+
+  if vim.g.lazygit_on_exit_callback ~= nil then
+    vim.g.lazygit_on_exit_callback()
+  end
 end
 
 --- Call lazygit