ENH: tag formatters

[story645]

Is it possible to provide a way to configure how tags are rendered in the badge separate from how they're defined in text. For example in-text tagging may include hierarchy ..tags:: domain:geometry but we'd like the rendered tag to display just geometry because we can show the hierarchy in a different manner. I'm thinking something like a callable:
Tag.set_formatter(lambda: x: x[:10])

attn: @esibinga