Added functions to set colors using RGB values

[nangahamandine]

In addition to adding the functions based on the requirements of issue #46 , I made some updates to the previous code base based on a syntax error in the main branch. I also updated the circle.ml file to use the Joy library in order to test the changes made. For some reason, after every change, I still get only the black circle displayed and not the colored circle.

Please review these changes. Would love to get your feedback

[nangahamandine]

Okay, I'll make the changes. Thank you @nikochiko

[nangahamandine]

I've updated the code:|

• To make fill and stroke non optional and the default for fill is transparent while for stroke is black. The shapes can now use the default values
• I have created a new type for color. It looks like Transparent | Fill of int * int * int | Stroke of int * int * int as @Sudha247 proposed which can store this information in the same field in a shape.
• This shape.mli file has been updated accordignly.
• I created a new file called colored_circle to test the changes and the circle displays perfectly fine with the colors that I defined.
• I've updated the rest of the code accordingly.

[nangahamandine]

I've updated this PR based on the requested changes by @nikochiko . Please review