Add support to set color for plots

```cpp
#include

int main()
{
const int n = 10;
af::array X(n);
af::array Y(n);
for(int i = 0; i < n; i++)
{
X(i) = i + 1;
Y(i) = (i + 1) * (i + 1);
}
af::Window win;
win.setColorMap(AF_COLORMAP_BLUE);
while (true)
{
win.plot(X, Y);
win.show();
}
}
```

The colour of the line is still red and doesn't change.