Update code throughout linterReporter for better style and performance.

Mostly fixes from previous PRs of mine. Some (seemingly) better comments and ways of doing things.

@MiguelCastillo: I'll add comments on the lines where I want to better document why the change was done.
I would guess that LoDash would optimize this better than this (it seems to, see https://github.com/lodash/lodash/blob/master/lodash.js#L3530)