Add JS guideline: "Avoid abbreviations"

[mcmire]

Abbreviations are tempting because they make it easier for the writer. But code should be written for the reader, not the writer (to paraphrase some quote I read that I'll need to find). Generally, abbreviations make code harder to read, and can also lead to ambiguity. There are some exceptions such as repo, however, which are generally always understood to mean "repository".