fix: console log Unknown language: ""

highlight.js throws an error for option `{auto: false}` if no language
is given. As an ugly side-effect this error is logged via console.log.
Setting the language in thoses cases to `plaintext` prevents this.

index.js

@@ -10,7 +10,7 @@ const maybe = f => {
 
 // Highlight with given language.
 const highlight = (code, lang) =>
-  maybe(() => hljs.highlight(lang, code, true).value) || ''
+  maybe(() => hljs.highlight(lang || 'plaintext', code, true).value) || ''
 
 // Highlight with given language or automatically.
 const highlightAuto = (code, lang) =>

test.js

@@ -1,4 +1,4 @@
-const { equal } = require('assert')
+const { strictEqual: equal } = require('assert')
 const md = require('markdown-it')
 const highlightjs = require('./')