Added support for histogram matching

[psychosomaticdragon]

I've added support for the histogram matching algorithm as described in the following paper https://arxiv.org/abs/1701.08893 When used in conjunction with gram or mean based style transfer, it helps stabilize style textures within the generated image. When used on its own, i've found that it preserves content image features much better than gram-based style transfer does, while still reproducing a style transfer of high quality.

Currently only implemented as a CPU based method, as creating an openCL or CUDA kernel for the histogram matching algorithm is non-trivial, and will probably take me a while to complete.