Add ability to override constraint options

Assuming that we always want to use NSLayoutFormatAlignAllCenterX/Y causes certain layouts to become impossible to create. This minor change adds the ability to optionally pass the options for each constraint individually, as so:

        layout.vertical '|-padding-[headline]->=0-[byline]-padding-|', NSLayoutFormatAlignAllLeft
        layout.horizontal '|-[headline]-|'

The default behavior is unchanged, and this change should be fully backwards compatible.