Allow to use custom Text component

[slorber]

import { Text } from 'react-native';

This is a nice default, but one might want to use another variation of this component

There is a context.textComponent option that you should consider using in case the default Text of React-native is not what the user wants to use (this is my case, because I use Text component from Glamorous)

See https://github.com/yahoo/react-intl/blob/master/src/components/message.js#L59