Added custom colors to the CannyFilter and the WeakPixelFilter

I extended the two classes below to support custom coloring. If the filters are used as is, they default to black backgrounds and white foregrounds. If the user sets either or both of the colors mentioned below, the filter output is effected. 

GPUImageWeakPixelInclusionFilter:
Added GPUVector4 fillColor;
Added GPUVector4 pixelColor;

GPUImageCannyEdgeDetectionFilter:
Added GPUVector4 edgeColor;
Added GPUVector4 fillColor;