Add proper fenced code-block formatting

[tristandruyen]

Markdown defines tripple back-ticks (```) as delimiter for code blocks, like

this

As i use notes extensively to save shell snippets, hack out readme's and the like, them being in a "normal" non monospaced font is extremely inconvenient...

It looks like my proposed change implements this in a quite minimal manner, and as far as my testing goes, doesn't break anything.

[tristandruyen]

BTW, While implementing this, I found it quite difficult to get into the simplemd & codemirror stuff, expecially as simplemd seems to be unmaintained since 2 years...
Would a PR to replace the markdown renderer with a more recent, extendable & actively maintained renderer be welcomed?
Are there any big technical hurdles that i should watch out for while implementing this?

If not i would love to contribute a bit more extensively than this 3 line css change^^

[jkaberg]

@tristandruyen #204 😄

[korelstar]

Thank you very much for this fix!

Some help for enhancing the editor would be greatly appreciated. :-)
As @jkaberg wrote, there is already a discussion on alternatives to SimpleMDE (see #204). Firstly, we should decide for a good library, and then we can implement it in this app. You're welcome on participating in this discussion and, later, do the implementation :-)