Optionally provide markdown descriptions in the JSON specification

[WillHannon-MCB]

The only way to provide a markdown description of your data is with a URL link to a markdown file. It would be nice to have the option to include a markdown description in the JSON file as well. This turns out to be a complicated problem due to the design of the tool.

Currently, there is only one markdown description per 'visualization'. I think this is reasonable. A single visualization can have multiple datasets, but there should be a coherent theme between them that don't require a new description whenever the dataset changes.

However, when configure-dms-viz creates a JSON specification it only makes a single file per dataset. If you have multiple datasets, you can combine the individual JSON files using some simple python. The issue is that there isn't a way to specify a 'top-level' parameter, like a markdown file or link to a markdown file, for a set of joined datasets.

The solution to this is probably to add another command to configure-dms-viz that joins datasets together and gives the opportunity to specify top-level parameters like a markdown file.