Provide an onload callback setting

[nfrid]

Case: I want to check if a buffer is 'gitable' and change signcolumn option to 'yes' locally. But VGit loads asynchronously and I can't just create autocmd on BufEnter.

It would be great if it has an option like 'onload callback' where user can define what to do when VGit is fully load itself into the buffer.

Alternatively, it can just have an option to switch signcolumn on/off depending on the vgit status.