Bind benchmark libraries to fixed, standard colors in plots

[stillyslalom]

Easier to compare from run to run, or between different folks' benchmarks, if MKL is always red, OpenBLAS is always black, etc.

[chriselrod]

I would also like it to now always set the limits at an integer log10 value. That creates unnecessary and excessive amounts of whitespace much of the time.

Do we want to bike-shed colors? I say MKL should be blue (Intel), and Octavian should take red (color of Roman standards). Everything else from Colors.distinguishable_colors because I'm lazy.

[mcabbott]

You could consider also doing Julia = solid, C call = dashed, or some code like that.