Add support for conversion of Trello attachments

Issue type

Feature request.

Explanation

It is possible to support transfer of attachments. Example trello api request that I've end up with to get all needed data from attachments:

https://api.trello.com/1/boards/{BoardID}/cards?key={Key}&token={Token}&attachments=true&attachment_fields=mimeType,name,url&checklists=all&fields=desc,closed,labels,name,shortUrl

Implementation suggestions

Would be nice if there would be an option to include custom format on how attachments are converted based on mime type (for example to embed videos/images with ! before an attachment so it gets embedded). Otherwise attachments should be transferred as close to Trello format as possible meaning they get inserted at the bottom of the description.

Resources

Trello API cards as nested resource

Example