Allow gatsbyRemarkPlugins string reference (#326)

Author: alexandernanberg

examples/custom-remark-plugins/gatsby-config.js

@@ -22,16 +22,16 @@ module.exports = {
         ),
         remarkPlugins: [require("remark-toc")],
         gatsbyRemarkPlugins: [
-          { resolve: `gatsby-remark-katex` },
+          "gatsby-remark-katex",
           {
             resolve: "gatsby-remark-images",
             options: {
               maxWidth: 1035,
               sizeByPixelDensity: true
             }
           },
-          { resolve: "gatsby-remark-autolink-headers" },
-          { resolve: "gatsby-remark-prismjs", options: {} }
+          "gatsby-remark-autolink-headers",
+          "gatsby-remark-prismjs"
         ]
       }
     },

examples/docs/src/pages/api-reference/options/gatsby-remark-plugins.mdx

@@ -33,12 +33,3 @@ gatsbyRemarkPlugins: [
   }
 ];
 ```
-
-# Not
-
-Using a string reference is currently not supported for `gatsbyRemarkPlugins`. (A PR would be accepted for this).
-
-```js
-gatsbyRemarkPlugins: [
-  `gatsby-remark-images`,
-];

packages/gatsby-mdx/utils/get-source-plugins-as-remark-plugins.js

@@ -41,6 +41,7 @@ module.exports = async function getSourcePluginsAsRemarkPlugins({
 
   // return list of remarkPlugins
   const userPlugins = gatsbyRemarkPlugins
+    .map(plugin => (_.isString(plugin) ? { resolve: plugin } : plugin))
     .filter(plugin => {
       if (_.isFunction(require(plugin.resolve))) {
         return true;