Update index.js to iterate named exports
Add <HTMLMessage> component

This adds support for an <HTMLMessage> component that will work with
ICU messages that contain HTML in them. It first HTML-escapes all
string props before formatting the message, then it uses
`dangerouslySetInnerHTML` so React won't try to escape the contents.

Closes #26
Clean-up components' render() and displayName
Add support for configured tagName to HTMLMessage
Update example to render using components