Add line numbering in code snippets by default

[lukaszstolarczuk]

We could add line numbering for code snippets/highlighting as default param in hugo.

Right now, because of using syntax, e.g. like:

```c++

we lose line numbering. We can make it appear by using syntax like:

{{< highlight C "linenos=table" >}}
{{< /highlight >}}

But then the code highlighting does not work well in IDE (at least in mine 😉 ).

ref. #270