[MLton] adding -ann switch

[Stephen Weeks]

I'm wondering if we could replace the following command-line switches

    -allow-export {false|true}
    -allow-import {false|true}
    -dead-code {true|false}
    -sequence-unit {false|true}
    -warn-match {true|false}
    -warn-unused {false|true}

with a single switch

	-ann <ann>

so that instead of writing

	-allow-export true -sequence-unit true

one would write

	-ann 'allowExport true' -ann 'sequenceUnit true'

or even

	-ann 'allowExport true, sequenceUnit true'

This way, we only have a single switch, we get to share the
documentation for annotations, and when we add new annotations, we get
access to them on the command line for free.