initial idea for how using tags will look

/cc @ianstormtaylor this is working, using the new tags dsl idea

it would get cleaned up / simplified more once we can remove `.load` entirely (after we merge the other 2 prs)
https://github.com/segmentio/analytics.js-integration/blob/a969a5262c93d019d4bb895cd0d76cf358c5b726/lib/protos.js#L253