Generate graph of memory usage over time

[itamarst]

ala https://github.com/astrofrog/psrecord, and include in HTML report with peak marked.

This would help prevent confusion, e.g if my diagnosis of #192 is correct.

[itamarst]

See https://docs.rs/plotters/0.3.1/plotters/ and https://docs.rs/lttb/0.2.0/lttb/