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.
Inline `isObject` #cosmetic

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
```