Markdown support in MJML/HEML emails

[adamquaile]

Consider how to add markdown support and where it would occur as a pipeline stage. See https://github.com/SparkPost/heml/issues/8 as an example of where it'd be useful.

In the context of an email or part of a document we'd need to delimit it, perhaps by a tag, like <markdown> or maybe by a generic pattern match in case it's not HTML.

As this would be implemented in Pipeprint, it should not conflict with the other use-case, whole documents. For example, a markdown->html->pdf pipeline.