FR: Support constant/user-supplied data

[znewman01]

Thanks for your work on this tool!

I'd like to be able to draw an extra line on the plots generated by cargo criterion. A couple use cases I can imagine:

• I know some theoretical maximum performance that I'm trying to get close to (why I bring it up). For example, my function processes something at X bytes/s, and my CPU is capable of Y bytes/s. I'd like to get a sense in the combined plots of how close I am.

  For this, I'd just like to provide a single throughput rate and have a straight line drawn on a (hypothetical) throughput plot.

• I have some objective I'm trying to hit (timeout, etc.).

  This would be a horizontal line on a benchmark; again, I'd like to just provide a number.

Vaguely related to #9, #10.

Potential workarounds:

• Add a "comparison function" sleeps for the appropriate time. I tried this and it's really noisy (not shocking).
• Hand-generate the data and sticking it in the appropriate location in target/criterion. I'm thinking about doing this.