add support for daily benchmark jobs

[jrevels]

This PR contains what I worked on this weekend to prepare @nanosoldier to run daily benchmark jobs (see JuliaLang/julia#13893).

Changes:

• slightly improved job submission phrase validation/tests
• raw benchmark data + logs are now uploaded as tarred JLD + text files
• add isdaily keyword argument to submission phrase that modifies the job to...
  • format the report folder name as daily_YYYY_MM_DD, using the current day's date
  • compare the current day's results to the previous day's results (if the previous day's results exist; it'll look back up to 30 days).

TODO:

• do a test deployment with these changes
• set up a cron job that causes @nanosoldier to regularly submit CI jobs to itself using the isdaily keyword argument

cc @tkelman

[tkelman]

can't wait