Suggest auto-installation if plug.kak not found

[pickfire]

Prior art https://github.com/junegunn/vim-plug/wiki/tips#automatic-installation

[jdugan6240]

There's a great snippet that does exactly this in this thread here: https://discuss.kakoune.com/t/snippet-that-automatically-installs-plug-kak/1399/4.

It essentially works by checking if the plug.kak directory exists, and if not, running git clone to grab it and source it.

If you want to do this for the other plugins plug.kak manages, it can be handled in a similar way, substituting the plug.kak plugin for whatever plugin is needed in the snippet.

[pickfire]

Should we put it in README?

[jdugan6240]

I'm not the maintainer of this repository, so I would only be able to submit a pull request, but I'd think adding a "Tips and Tricks" section to the README would work fine. We may even be able to resolve #15 as well while we're at it.