Validate plugin options

[zachleat]

Some confusion about const { feedPlugin } = require("@11ty/eleventy-plugin-rss"); versus const rssPlugin = require("@11ty/eleventy-plugin-rss"); in #56 might merit some better error messaging when invalid options are passed to plugins.

[harrislapiroff]

I ran into this confusion as well, so +1 on an error message here :)