Feature: allow multiple default locales, for defaultMessage

[VictorBaron]

New Feature

What is the current behavior?:
When running the script, it will insert the default message for the default locale only, and an empty string for other locales.
If i don't update immediately my locale files, react-intl fails and displays the id

What is the expected behavior?:
The purpose of the default message is to be a fallback.
I would want to export it by default, instead of an empty string

Suggested solution:
Either allow an array of default locales as a param, or a specific option that allows to extract the default message to every locale file