rule proposal: method names

[gabro]

proposal: method names should never start with _. I mostly agree with AirBnb, after reading this discussion airbnb/javascript#1024

We could use this brand new eslint plugin to enforce it https://github.com/buildo/eslint-plugin-method-names/tree/master 💯

@buildo/frontend

[FrancescoCioria]

For me it's ok, but we'll have to fix a lot of code for this (ex: I did it once today :P)

[gabro]

by the way, I'm proposing this as a core rule on ESLint, let's see whether it gets accepted.

eslint/eslint#7065