Add an interactive mode

[glandium]

In some cases, like performance regression, it's hard to write reliable automated tests to drive the bisection. It would be great to have a git-bisect like interactive mode allowing to run commands manually and then tag whether the tested version was good or bad.

[est31]

Good example of something hard to automate is rust-lang/rust#49336 ... I guess I have to resort to using rustup for this.