plots: CLI support for flexible plots

[dberenbaum]

There is no way to configure flexible plots from the CLI.

dvc plots modify could be updated to take a plot name key as a target instead of only a file, and to support modifying the other fields to support file paths, multiple axis values, etc.

dvc plots add could be added to support configuring a standalone plot from the command line. It could look a lot like dvc plots modify except that the target won't already exist.