[plt-scheme] mztext replacement

This is great!

>   * It seems to me that just the above is enough for the usual use of
>     a saner tex front end.  Do you see any extra bells and whistles
>     that are needed?

This is mostly due to my unfamiliarity with Scribble: How would you
replace defcommand?

> 
>   * I originally thought to make a command-line interface that is
>     similar to mzpp/mztext -- but I think that there is almost nothing
>     that this will get that using plain mzscheme doesn't.  That is,
>     you can use it with
> 
>       mzscheme foo > bar
> 
>     which will spit out the "preprocessed" contents of "foo" into
>     "bar".  Are there practical uses that need more than that?

No, this is just fine.

~ Aleks