Add machine comparison list for published results

[qunaibit]

ASV tracks performance regression and improvements per machine and python versions. However, tracking performance on multiple configurations/optimizations options for python builds can only be done as separate machines configurations. Though, comparing machines against each other is only visible per benchmark view.

This PR will provide a comparison list for machines against each other. It can, also, exports the comparison list to markdown format (example).

[datapythonista]

Thanks for the contribution. Probably better if you implement this as a third-party package. Not only because the copyright, also because asv is mostly unmaintained, and the last thing we want is even more code to maintain in it.

[datapythonista]

Surely not merging with this.

[mattip]

@qunaibit would you like to continue with this?