Add missing class="hljs" to pre tag

[stami]

Highlight.js styles have a root .hljs class that contains root text color and background color.

This PR adds that class to the outputted HTML and fixes background styles not applying.

Easiest to see when using a dark theme: before the background was white (or other inherited site background) but after this addition the code block uses the background color of the given style.

Before:

After:

I'm using Atom One Dark styles there.

[alex-ross]

@stami and @stami-fb do you know why this is needed? According to Highlight JS usage page this should not be required.

[stami]

Well, I couldn't figure out how to get the theme's background color to apply without adding the class. 😁

I guess in the usage page the hljs.highlightAll(); call dynamically searches for any <pre><code> tags from the page and adds the class there, but in this static site plugin it is not done.

I'm confused, does this plugin work for you without the hljs class? Try with a theme that has different background color than your website otherwise has to see it. In my example (the screenshots) I'm using Atom One Dark theme and it has the dark background set in that .hljs block. Without my addition, that class is found nowhere in the generated static html and so the background is left white (the upper screenshot). 🤔

[alex-ross]

@stami hmm... You are correct. In my own site the site has the same background as i expect in the colorscheme.

In worst case, what could go wrong adding it twice 😉.

I will merge this and release a 1.0.2 version very soon with this improvement. Thanks for your contribution.