No grade bug fix

[diego-codes]

This PR addresses the issue submitted in the colors repo: IBM-Design/colors#53

Problem

The assumption is that if I use the color function without a grade argument, I should get the core grade of the color I am requesting:

.foo {
  color: color('blue'); // Core grade is 50, so the color to return should be #2d74da
}

The default value for the $grade parameter in the color() function is set to 0 which currently no color has this as a possible grade. Updating the default $grade value to a number that does in fact exist in all palettes doesn't work because each color has a different core grade associated with it.

Solution

The solution I have come up with is to add one more grade item to each color map in the output _bee-palette.scss with the key of core and value of whatever that palette's core grade hex value is. This way we can set the default value for $grade in the color() function to 'core' and fulfill the expected behavior of calling the color() function without supplying a grade.

Along with this solutions, I took the liberty to:

• Update the .ase, .json, and .scl output templates to include the core grade value.
• Improve (fix?) error handling inside of the color() function to throw an error if the user provides a color or grade that does not exist as well as a warning if they're outside of the $alpha channel value range.
• Lastly, added unit tests for the color() function using True and Mocha! Sadly, I don't think there is a way to test for errors which would have been nice.