[MLton] Added completion to esml-mlb-mode

[Stephen Weeks]

> >   forceUsed {false|true}
> 
> This may not be what you want. The forceUsed annotation
> doesn't actually allow an argument:
...
> An alternative (perhaps better?) would be to change the syntax
> of forceUsed to require an argument (using makeBool).

Sounds good to me.