escape text in code block when lang is 'text' or not specified

[lazzzis]

Content in my markdown file:

Run vuepress dev, it throws an error:

Run vuepress build, it doesn't throw an error. But the page is not rendered as expected:

---

I guess the cause is that code isn't escaped, when lang is 'text' or not specified.

vuepress/lib/markdown/highlight.js

Lines 8 to 10 in 86d4055

	function wrap (code, lang) {
	return `<pre v-pre class="language-${lang}"><code>${code}</code></pre>`
	}

Since prism would automatically escape code:

> prism.highlight('<scheme>', prism.languages['html'], 'html')
'<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>scheme</span><span class="token punctuation">></span></span>'

A quick solution might be:

const escapeHtml = require('escape-html')

function wrap (code, lang) {
  if (lang === 'text') {
    code = escapeHtml(code)
  }
  return `<pre v-pre class="language-${lang}"><code>${code}</code></pre>`
}

This solution works well both on vuepress dev and vuepress build. But I don't know if this is a good solution

[ulivz]

PR welcome.