Directives for specific output types

[mgalloy]

There should be directives for output types that pass the embedded text directly to the output or skip over if that is not the current output type.

For example, the following should make a table in HTML output, but not show up in other output types:

; .. html::
;
;   <table>
;     <tr><th>Type</th><th>Format</th> </tr> 
;     <tr><td>type 1</td><td>format 1</td> </tr> 
;     <tr><td>type 2</td><td>format 2</td> </tr> 
;   </table>