export-queries: Include more metadata in the head of the file

[susodapop]

The export-queries script was added to redash-toolbelt from an existing script. It writes for metadata to the top of each file in addition to the query text:

• Name
• Data Source
• Created By
• Last Modified At

It would be nice to write this metadata in a machine readable format (YAML probably). And to write all of the metadata available including:

• Description
• Tags
• Last Executed At
• Created At