Add function-disallowed-list rule

[pamelalozano16]

Fix #422

[kristerkari]

Looks good!

There are still some extra module syntax that we should add testcases for. Have a look at the function-no-unknown rule test cases here:
https://github.com/stylelint-scss/stylelint-scss/blob/master/src/rules/function-no-unknown/__tests__/index.js#L61-L82

[kristerkari]

Oh, I see. The rule does not actually care about the namespace, it just checks the function name.

I guess that it works, but I guess that there is an edge case where you would have the same function name locally and inside a namespace. That should not be too big of a problem tho, and the namespace support can be added later on.

[kristerkari]

Looks good now 👍

Thanks for the contribution!