Adding syntactic sugar to reduce

[NickHeiner]

This PR enables the following two syntaxes:

var concatted = _.reduce([['foo', 'bar'], ['baz'], ['batz']], 'concat');
// => ['foo', 'bar', 'baz', 'batz']

and

var product = _.reduce([2, 4, 8], '*');
// => 64

If you like them, I'll also add them to reduceRight. Tests and documentation are included. I can break these out into two separate PRs if you prefer. I tried to follow the existing style conventions but let me know if I can make anything look nicer. I wasn't quite sure where the utility methods should live, so I opted for "as close as possible to where they are used".

[jdalton]

We handle callback sugar through _.createCallback which allows ".where" & ".pluck" though _.reduce and _.reduceRight currently don't leverage it. I can make them use it so that further customizations are possible.

[NickHeiner]

Even if _.reduce and _.reduceRight leverage _.createCallback, _.createCallback will have to be changed to use this syntax, right? Is this approach so far off that it looks nothing like the way you'd like it done? Or do you not think this is a feature worth having?

Also, apologies for the failing travis build. I was just testing by running node test/test.js, but it looks like I have some compatibility issues.

[jdalton]

Even if _.reduce and _.reduceRight leverage _.createCallback, _.createCallback will have to be changed to use this syntax, right?

Yap, but that's why it exists :)

[NickHeiner]

Would you be amenable to a PR changing _.createCallback to use the new syntax? :)

[jdalton]

Not at the moment, I've made it customizable/wrapper for these kinds of things. Though I will make _.reduce and _.reduceRight use it.

[NickHeiner]

Ok, sounds good.

[lock]

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