usage message

[Matthew Fluet]

> Here is what the new usage message will look like.  I decided to go ahead with
> the -keep g, since it doesn't seem too useful to be able to decide independently
> to keep the generated c or S.  I also changed -stop to use g as well.

I like -keep g rather than two -keep's.  I also agree with Henry that the
idea of keeping the generated code is something that is likely to come up.

> 
> usage: mlton [option ...] file.{cm|sml} [library ...]
...

Looks good to me.  I was only adding "Experimental" to keep the
distinction between the C backend and the x86 backend options.  I think
the "?" is fine.