Allow plugins to access pathPrefix

[zachleat]

module.exports = function(eleventyConfig) {
  eleventyConfig.addPlugin(function(eleventyConfig) {
    // same as eleventyConfig.dir
    console.log(eleventyConfig.pathPrefix);
  });
};

Found a performance issue with the Eleventy Navigation plugin that was calling the url filter way too much, so we’re adding a new code path to bypass the url filter if pathPrefix is not used.

Eleventy:Benchmark Benchmark    312ms   9% 13934× (Configuration) "url" JavaScript Function +0ms

Shipped with @11ty/eleventy-navigation@0.3.4

[zachleat]

Relevant to 11ty/eleventy-dev-server#37 (comment)

[zachleat]

Shipping with 2.0.0-canary.15