Warnings for non-native properties

[ksafranski]

Currently there's no way to tell if an invalid property is supplied, so for instance; if you misspelled modifer: 'someVal' there's no indicator that the property won't be acted upon. Since we want to allow custom properties my suggestion is to "throw" a warning if the key in a definition is not recognized, then create a obey.customProps(<array|string>) that allows for centrally declaring custom props to prevent the warning.

[ksafranski]

Closing this as we want to leave the opts open for custom props in schema defs without dev's needing to jump through hoops