rename `csv_url` to `data_url`?

[7yl4r]

In addressing #21 I found it easiest to abuse the csv_url column for non-csv data urls. Examples:

• the new plotter embed_html uses the csv_url column for a link to the html
• the new plotter include_graphic uses csv_url for a link to the image

This is a little confusing because the column name csv_url implies that the link is the csv data. Ways we could resolve this:

1. rename the column csv_url to data_url
2. use a different (additional) column for embed_html and include_graphic