replaceColor does not accept RGB array of 4

[greghesp]

If I run getColors on my json, I can see it returns arrays of 4, assuming the 4th is the alpha layer.
replaceColor however, only accepts an array of 3

[vladotesanovic]

I had same issue as this library didn't work out of the box for my lottie files, I had to change source code a bit to support it.

Here is the PR for supporting alpha channel:
#51