Write CNAME file if custom domain

Custom domains in GitHub pages require a CNAME file. This file is automatically created for you if you add a custom
domain to the GitHub pages section of the "Settings" area in a GitHub repo.

However, the way we push to GitHub, we remove all files from a `gh-pages` branch before building and pushing the new
content on a rebuild. This includes removing the CNAME file, even if it was autogenerated when changing the setting.

So, just always create a CNAME file if the domain is a custom domain.

lib/server/generate.js

@@ -385,6 +385,19 @@ function execute() {
       fs.copySync(file, targetFile);
     }
   });
+
+  /* Generate CNAME file if the domain is custom */
+  if (!siteConfig.url.includes("github.io")) { // github.io urls are not custom
+    let cname = siteConfig.url;
+    let targetFile =
+      __dirname +
+      "/../../build" +
+      "/" +
+      siteConfig.projectName +
+      "/" +
+      "CNAME";
+    fs.writeFileSync(cname, targetFile);
+  }
 }
 
 module.exports = execute;