Generalized raw attributes.

````
% pandoc -t native
```{=ms}
.MACRO
foo bar
```
^D
[RawBlock (Format "ms") ".MACRO\nfoo bar"]
````

````
% pandoc -t native
Hi `there`{=ms}.
^D
[Para [Str "Hi",Space,RawInline (Format "ms") "there",Str "."]]
````

````
% pandoc -t native
~~~ {=ms}
.MACRO
foo bar
~~~
^D
[RawBlock (Format "ms") ".MACRO\nfoo bar"]
````
