feat: Add rule checking Prism languages

The linter now checks that fences have languages and that those languages are
supported by PrismJS. The linter has been extended over the guide with its own
set of rules that only validate code fences.

tools/lint-guide/lint-guide.js

@@ -0,0 +1,12 @@
+const lintRules = require('./.guidelintrc');
+const linter = require('../linter');
+const argv = require('yargs').argv;
+
+const isMDRE = /.*\.md$/;
+
+const lint = linter(lintRules);
+
+const files = argv._.filter(arg => isMDRE.test(arg));
+files.forEach(path => lint({ path: path }));
+
+module.exports = lint;