Markdown cleaner does not clean indented code blocks

[averms]

Backtick code blocks are stripped but indented ones are not. They are part of the CommonMark specification so I think it is reasonable for them to be supported.

Example:

Here are a few words.

    <pre class="lua"><code>print 'Hello World!'</code></pre>

Here are some more.

This gives a lot of errors when run with textidote --check en_us test.md.