Mark `memo` optional in `reduce` and describe.

The `memo` parameter of `reduce` is optional, but it was not marked as such. No description was given as to its behavior when omitted.
Merge pull request #1759 from janaspage/patch-1

Mark `memo` optional in `reduce` and describe.
get rid of editorconfig.
Tweak to allow ESLint to pass cleanly.
Fixes #1767 -- expose lookupIterator as _.iteratee.
Enable pluck like (and falsey) iterators in uniq
Merge pull request #1744 from megawac/uniq-falsey

Enable pluck like iterators in uniq
#1767 -- s/iterator/iteratee
#1767 -- s/iterator/iteratee in docs.