The kakoune editor has a plugin mechanism and several plugins are
already packaged under `pkgs.kakounePlugins`. However, adding these
packages to `home.packages` is not enough: the `kakoune` package needs
to be configured with the list of plugins to include, so that they get
sourced on start-up.
We add a `programs.kakoune.plugins` option, analogous to
`programs.vim.plugins`.
The change is backwards compatible since `pkgs.kakoune` is defined as
wrapKakoune kakoune-unwrapped { };
and `wrapKakoune` defaults the list of plugins to empty.
PR #1356
