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" ]
````