Allow markers for problematic Rust code

[atifaziz]

This PR adds Ferris from The Rust Programming Language and as used there to mark-up problematic Rust code. In fenced code, one of the following additional tags may be used after the rust language:

Tag	Ferris	Meaning
does_not_compile		This code does not compile!
panics		This code panics!
not_desired_behavior		This code does not produce the desired behavior.

For example:

```rust,does_not_compile
// code for illustration, but that does not compile 
```

Unfortunately, due to relative path use in ferris.js, the images folder (img) is (symbolically) linked into every folder.