Add mimalloc to benchmark charts?

[joshtriplett]

#101 was closed, but it doesn't look like the charts (either in rpmalloc's README or in rpmalloc-benchmark's README) were updated to include mimalloc.

I would be very interested in current numbers here, for the purposes of choosing between mimalloc and rpmalloc.

[qtxie]

IMHO, The design is quite similar to mimalloc.