This change allows to pass custom packages into the `vim.plugins`
option.
Additionally this adds a deprecation warning and an error message if a
vim plugin is not present. This is an improvement because the user
gets instant feedback, when a plugin is not found.
This option gathers basic Vim options into a single place. The idea is
to allow many options without making the Home Manager documentation
too verbose.
This also deprecates the options `programs.vim.lineNumbers` and
`programs.vim.tabSize`.
Fixes#69.
This adds a readonly package option which will be set to the resulting
configured vim package, so it can be refered to by other configuration.
An example would be home.sessionVariables.EDITOR =
config.programs.vim.package + "/bin/vim".
