Reduce an array to unique values, optionally into a separate array.
Example
> const unique = require('reduce-unique')
> const arr = [ 1, 3, 8, 3, 1, 2, 1, 9, 3, 3 ]
> arr.reduce(unique)
[ 1, 3, 8, 2, 9 ]
> arr.reduce(unique, [ 'one', 'two' ])
[ 'one', 'two', 1, 3, 8, 2, 9 ]
> arr.reduce([ 3 ])
3© 2016-18 Lloyd Brookes <75pound@gmail.com>. Documented by jsdoc-to-markdown.