Support relative size assertions

[simonihmig]

Currently the addon supports only absolute size assertions. From the beginning of working on this I had the idea that supporting relative assertions (e.g. max increase of 1%) would be really cool, but semantics and implementation is not clear to me yet:

• Compare the current size to what?
  • previous commit? (useless with a git style of having many small commits)
  • master branch
  • previous release (git tag matching /v\d+\.\d+\.\d+/)
• how to get that previous size?
  • recalculate each and every time by checkout out the old git commit and running the build again (slow)
  • somehow persist the history of calculated sizes?
    • but where? In git, but how to automate that this gets checked in? Somewhere externally? (don't want to operate any kind of infrastructure for it, eventually privacy issues)
    • would allow some even cooler features, like a complete size history / charts.

[makepanic]

I don't think we can easily persist the information over multiple builds without creating data which has to be committed.

With addons, we could check the published artifact to compare versions.