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" ]
````