Warn if context supplied without corresponding childContextTypes

[WickyNilliams]

I think it would be useful to warn/error if context option is supplied without corresponding childContextTypes option (and possibly vice versa, though i imagine that's far less likely to occur).

In the last few days, a teammate of mine and someone on reactiflux have both made the mistake of omitting childContextTypes. So it seems like an obvious place to guide developers in the right direction.

[ljharb]

Related to #664.

[ljharb]

Now that #664 is merged, if you fail to provide childContextTypes, then children simply won't get your context - so this should no longer require a warning.