add `im_from_matlab` and `im_to_matlab`

[johnnychen94]

This provides a more convenient MATLAB interface compared to the composition of PermutedDimsArray, colorview, etc (CRef #178)

Although our naming guidelines recommend not adding im_ prefix, I feel from_matlab is somehow too generic a name, especially when the input is a raw numerical array. For this reason I use the long verbose name im_from_matlab, users could add their own alias if they want, e.g., const im2jl = im_from_matlab

closed #170