Replace `mkNeovimPlugin`'s `installPackage` parameter with a new
internal module option: `packageDecorator`.
The option is a function that applies some transformation to
`cfg.package`'s value before it is installed.
Added some more upfront details about the plugin's default settings.
Lots of people assume just enabling the plugin will give them the
behaviors of highlighting they expect, but the plugin does not default
this to be the case.
This commit adds a `plugins.<name>.luaConfig` section controlling the
plugin specific configuration.
The section contains the internal `init` option, containing the plugin's
initialization code.
It also contains the public `pre` and `post` options, that allow to add
code before & after the `init` section
Finally, it contains the `final` option, being the concatenation of the
three previous options.
