Add spec text for handling for `locale` and `usage` conversion options

[eemeli]

As noted by @waldemarhorwat in #108, we're missing an AO in the proposed spec changes that'll handle the locale and usage conversion options. It'll probably be an AO called from GetAmountConvertToOptions that'll be overridden in ECMA-402 to look up the data from the CLDR units.xml.