Expose Full Line-Length Diff Highlighting

Enabled a config param to the remark-prism plugin that allows Diff code blocks
to have the entire diff'd line be highlighted

Author: jon-sully

packages/gatsby-remark-prismjs/README.md

@@ -79,6 +79,13 @@ plugins: [
             // Add additional HTML escapes by providing a mapping
             // of HTML entities and their escape value IE: { '}': '{' }
             escapeEntities: {},
+            // Enables full-line highlighting on diff-code-blocks (see below)
+            // Disabled by default
+            // To use it, add the following line in gatsby-browser.js (after
+            //   the selected color scheme):
+            //   require("prismjs/plugins/diff-highlight/prism-diff-highlight.css")
+            // Not currently enable-able on a per-block basis
+            showDiffHighlight: false,
           },
         },
       ],
@@ -392,12 +399,28 @@ unless explicitly overridden by the `promptUser` and `promptHost` options in the
 
 ### Diff code blocks
 
-You can specify language for `diff` code blocks by using `diff-[language]` to enable syntax highlighting in diffs:
+You can specify language for `diff` code blocks by using `diff-[language]` to enable syntax highlighting in diffs. Canonically, when writing code _diffs_, the code should be indented by two characters. If the code is being changed, the first character should be a `+`, `-`, or `!` otherwise left a space. The second character should always be a space. Git-style file-location coordinates need not be indented.
 
 ````
 ```diff-javascript
+@@ -4,6 +4,5 @@
+  const doTheThing = aParm => {
++   console.log(`Parm is: ${aParm}`)
+-   console.log('Hit this function')
+!   console.log('Rubber duck debugging anybody?')
+    return true
+  }
+```
 ````
 
+The default rendering style for diff code blocks will be a colored `+` or `-` symbol and _not_ full line-length highlighting. This is depicted in the **first** example here: https://prismjs.com/plugins/diff-highlight/
+
+This plugin supports a configuration parameter, `showDiffHighlight`, which will add full line-length highlighting to _all_ Diff code blocks used by your Gatsby site with the format: `+` -> Highlight green, `-` -> highlight red, and `!` -> bolded line. Simply pass the configuration parameter with `true` and add the following to your `gatsby-browser.js` (_after_ the require for your chosen theme):
+
+```js
+require("prismjs/plugins/diff-highlight/prism-diff-highlight.css")
+```
+
 ### Line hiding
 
 As well as highlighting lines, it's possible to _hide_ lines from the rendered output. Often this is handy when using `gatsby-remark-prismjs` along with [`gatsby-remark-embed-snippet`](https://www.gatsbyjs.org/packages/gatsby-remark-embed-snippet/).
@@ -520,6 +543,10 @@ throw our overflow and background on `.gatsby-highlight`, and use
 `display:block` on `.gatsby-highlight-code-line` – all of this coming together
 to facilitate the desired line highlight behavior.
 
+**NOTE:** This is _not_ the same as the Diff code-block full line-length
+highlighting for diff'd lines. Those "line highlights" work great beyond
+the above.
+
 ### Line numbering
 
 Because [the line numbering PrismJS plugin][7] runs client-side, a few adaptations were required to make it work:

packages/gatsby-remark-prismjs/src/index.js

@@ -17,6 +17,7 @@ module.exports = (
     noInlineHighlight = false,
     showLineNumbers: showLineNumbersGlobal = false,
     showInvisibles = false,
+    showDiffHighlight = false,
     languageExtensions = [],
     prompt = {
       user: `root`,
@@ -78,7 +79,11 @@ module.exports = (
     //
     // @see https://github.com/gatsbyjs/gatsby/issues/1486
     const className = `${classPrefix}${languageName}${
-      diffLanguage ? `-${diffLanguage}` : ``
+      diffLanguage
+        ? showDiffHighlight
+          ? `-${diffLanguage} diff-highlight`
+          : `-${diffLanguage}`
+        : ``
     }`
 
     // Replace the node with the markup we need to make