Remove unnecessary variable and else-if #cosmetic

Recommend --word-diff-regex='\w+|.' to view diff.

I first noticed that we were doing isObject() twice if an object is
passed as `def` in the first place, and in simplifying that I realized
we didn't even need `extensions` in the first place.

`for (var ext in def)` didn't make much sense so I changed the name of
the iteration variable too.

Note that the old minified byte count in the README was out of date, it
was originally 544, the savings are good but not *that* good.
Inline `isObject`

Recommend --word-diff (default regex, not '\w+|.'), and GIT_PAGER= so
that the line in the README won't word-wrap, e.g.:
```sh
$ GIT_PAGER= git show --word-diff
```