New chapter on speed

[wlandau]

• Mention https://github.com/ropensci/drake/blob/master/.github/ISSUE_TEMPLATE/bottleneck.md#benchmarks
• Describe all the little shortcuts that speed up make(): verbose = 0L, log_progress = FALSE, recoverable = FALSE, history = FALSE, session_info = FALSE, lock_envir = FALSE, log_build_times = FALSE.
• Walk through example based on https://github.com/wlandau/drake-examples/blob/master/overhead/static.R, before an after those shortcuts. Show flame graphs and describe how to get them with pprof.

[wlandau]

On second thought, this is not essential enough for the manual.