plots: better support power use cases

[wwwillchen]

• create an /out-${ISO8601-date}/ folder to avoid having to constantly clean /out/
  • log the selected folder
• specify a --out path, which can be an existing path, in which case plots gracefully adds runs (this should satisfy the --run-resume use case that @brendankenny suggested)
• use timestamp as the run-id (folder name)
  • this will enable users to run concurrent lighthouse sessions with the same out folder
• make analyze.js have a mandatory argument (the out folder)
• analyze the /out/ folder based on query params for the charts (supports the custom out dir workflow)