Next major version release

[ahutchings]

Now that React v16 is out, this seems like a good time to plan the next major version of emojione-picker.

#58 (Use PureComponent instead of shallowCompare) would be a good one to include as a since it requires a increasing the min version of React we support to 15.3.

#55 (Frequently used) also looks close to ready and would be a nice feature to incentivize upgrading.

@tommoor Anything else you'd like to see included?