Create 0.4

## 0.4

The main change in 0.4.0 is that we hooked up @jch's html-pipeline such that it runs a configurable set of filters over the main slide content. This means that standard things like auto-linking, mentions, and emojis are easily implemented.

Additionally-- 0.4 introduces a notation where you can use a "triple fence block" and pass in "notes" or "note" to create a Speaker Note when you're working in markdown.

See the CHANGELOG.md for additional details.