Add no-shadow to eslintrc

[eritbh]

Apparently no-shadow isn't part of eslint:recommended. It should be, but that's not up to me.

This adds no-shadow to our config. This rule reports an error whenever a variable name from an upper scope is reused in a lower scope. This prevents uncertainty in which identically-named variable is actually being referenced in code. builtinGlobals ensures that we aren't shadowing the names of built-in objects, and hoist: all makes the following invalid:

if (someCondition) {
	const foo = 1;
	// errors because `foo` is declared in the upper scope, even though it comes after this block
}

const foo = 2;

[eritbh]

hm, no kidding we didn't have this enabled before, travis reports almost 200 errors caused by this change. Marking as WIP until I get around to fixing those issues.

[eritbh]

abandoning this, something about our eslint environment is wonky and marking lots of things as shadowed when they clearly aren't.

Probably not worth revisiting this until after #198, because that will allow us to clean up our mess of global variables somewhat. So this is a long way off from being practical.