Add annotation feature from ToolDog

As discussed with @jmchilton, @hmenager and @inkuzmin during GCC, we would like to contribute to Planemo by adding the annotation feature of ToolDog[1] based on description from the bio.tools registry, which enables the annotation of existing tools with bio.tools metadata.

I would like to discuss here what would be the best strategy to include this functionality. My first suggestion would be to provide a new command like `planemo biotools_annotate` that would call the corresponding code in ToolDog. This means ToolDog (and therefore galaxyxml, CC @erasche!) would become a dependency of planemo, so I'd really like your opinion on this before starting to work on it. Any thoughts?

[1] see https://github.com/bio-tools/ToolDog