Provide ability to add own log command

- [x] I have fzf 0.30.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

I have been using fzf on the command line for a long time now and been using an integration for git parameters extensively. However instead of pure git log output I am using an alternative output formatter called git-foresta. For this I am using an alias instead of log.

It would be most useful for me if fzf.vim could call a different command alltogether. So in vim.vim commits ~line 1457 instead of assembling the command with the commits_log_options check for the existence of a complete command option and if present use that instead.