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.
Added support for the plugin's "advanced" config settings.
Removed the enum restriction and prefix concatenation, allowing anything
to be passed through, including raw lua: fixes#1675
The theme list is now much shorter and is included directly in the option
description.
Some general cleanup, in particular to `extraConfig` and `customColorschemeType`.
