Make indicator boundary customisable

[assignUser]

I just looked at kgoldfeld/simstudy#129 And I wondered if the users could be a able to specify a threshold himself for the icon. Does it always make sense to check if 0 is contained? One could argue that a ci that contains a 1% performance drop is still not enough to justify a 🐌 icon, so if the user specifies a custom value for null_boundary (Better name needed I think ), we check if the confidence interval overlaps with it.

Originally posted by @lorenzwalthert in #71 (comment)

[assignUser]

I agree, that would be good, maybe something like max_performance_loss or something.