Add the possibility to assign the colors in the yaml file when the urdf model is generated

[GiulioRomualdi]

Thanks to this PR it will be possible to add color to customize the link colors of the urdf

This is required to achieve robotology/icub-models-generator#216

cc @traversaro

[traversaro]

Great! Can you add the docs for the new parameter in https://github.com/robotology/simmechanics-to-urdf/blob/1eb31b4e2d86e091c59cbe3cfc62d1e5265d7d61/README.md#yaml-parameter-file ? Thanks!

[traversaro]

Probably it could go in the "Mesh Parameters" section: https://github.com/robotology/simmechanics-to-urdf/blob/1eb31b4e2d86e091c59cbe3cfc62d1e5265d7d61/README.md#mesh-parameters .

[GiulioRomualdi]

Documentation updated in cdc7d2c