Auto active substep

I think it's useful to let use active a substep after enter the step, so the first substep (or specified substep) is active at first, then can be hidden after. e.g. we can show a slogan at first, then replacing it with actually content.

I thought of some way:
1. add attribute `data-substep-active`/`data-substep-active=` to `step` element
2. add attribute `data-substep-active` to a `substep` element
3. add class `substep-auto-active` to a `substep` element

Start by tracing how impress.js currently handles steps and substeps, including any examples or tests that demonstrate substep visibility. Compare the three proposed activation mechanisms and define one precise behavior for the initial or selected substep and its later hiding. Done means the chosen behavior is documented and covered by an appropriate example or test.

Written by the indexing model from the issue text.