Add a `replicate` Array function

This function replicates each element of an array `n` times:

    _.replicate([1, 2, 3], 2) //=> [1, 1, 2, 2, 3, 3]

I find it useful for when you have to do something where two arrays have to
match or have to be the same length or where each element of one should match
each element of the other and so on. I don't know if it can be general enough to
be merged into Lodash's core.

I added a bunch of tests for the function too.