[Feature Request] Add support to show vim version

Nobody has claimed this yet.

When juggling between different installed versions or forks of vim, the default start screen is quite helpful in reminding on what version/type of vim is actually running. For example I have both vim and neovim installed. Hopefully the shell alias `vi` points to the one vim I think it does. In case I remember wrong, the start screen reminds me of what I'm actually running

See screenshots of default vim & neovim start screens here:

It's possible to add the version manually by using e.g. `startify_custom_header` or `startify_custom_footer` as shown in #449 . However this is quite some effort to do. I think it would benefit many if there was some `startify_show_version` option that adds the current version as stylished in the first line of `:version`.

That is, for the versions shown in the screenshot above, setting a future `let g:startify_show_version = 1` would include somewhere in the startup screen the string
* for vim: `Vim 8.2.2029`
* for Neovim: `NVIM v0.5.1`

These snippets shows how versions could be extracted:
https://gist.github.com/erikw/bd4c552b11733cd1ba9440095edaa4e8

Start by tracing how startify_custom_header and startify_custom_footer are handled, using issue #449 as the reference for existing customization. Review the linked extraction snippets and Vim's :version output. Done means a startify_show_version option displays the appropriate Vim or Neovim version string on the startup screen.

Written by the indexing model from the issue text.