This changes the parsing of the language string
in code examples so that unrecognized examples
are not considered Rust code. This was, for example,
the case when a code example was marked `sh` for shell
code.
This relieves authors of having to mark those samples
as `notrust`.
Also adds recognition of the positive marker `rust`.
By default, unmarked examples are still considered rust.
If any rust-specific tags are seen, code is considered
rust unless marked as "notrust".