Add a function to generate plain-text summaries of EML metadata

[cboettig]

A couple of options on how to do this:

• Provide in markdown formatting?
• More crudely: convert directly to YAML instead? as.yaml(xmlToList(eml.xml))

Ideal rendering would drop some of the less essential markup (e.g. stuff intended more for machines than people -- unit definitions, numeric types, etc).