Markdown: Better language detection for code blocks

[RunDevelopment]

This improves the language detection of code blocks.

I wanted to add support for R Markdown notebooks, but MD couldn't detect the language because the code blocks in RMD can have some meta info:

```{r pressure, echo=FALSE}
plot(pressure)
```

The language will now be the first word after ```.

I also added support for c++ and c#.