Default to translation key if not found in locale json

[zcoffin86]

Mirror behavior of react-intl when key requested is not found in locale json.
#141

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[cwtuan]

Hi, @zcoffin86
Thanks for the contribution.
However, this PR may not be accepted since this change the behavior in current API.
We recommend use defaultMessage for every message in case that the key is not found.
For example, intl.get('HELLO', { name }).d(`Hello, ${name}`)
You could also add warningHandler in init function to accumulate missing messages using third party services.