Add support for config in a separate file (.bundlesizerc)

[bniwredyc]

Do you want to request a feature or report a bug?
Feature

What is the current behavior?
bundlesize can be configured by package.json section or CLI

If this is a feature request, what is motivation or use case for changing the behavior?
I have a pretty large package.json and I want to put libs' configs in separate files like .jestrc, .prettierrc, .eslintrc and so on. I want to do it with bundlesize too like this:
https://gist.github.com/bniwredyc/16a2e95ac06be090561710b0d9423680

If approved I will implement it myself.

[siddharthkp]

Implemented in #316 and published in 0.18.0 🎉

Check out README for details