Add resistor-color-duo

[BNAndras]

Towards #223

I used an enum here so resistor-color and resistor-color-trio should also use similar enums for consistency. The intention is that a student works through these three exercises as a group, unlocking more complexity along the way.

Some tracks omit the test about ignoring additional colors. Exercism as a whole doesn't make a big deal of input validation except for a few specific exercises. If we choose to not include the test, then we can use two Color parameters instead of the array.

[0xNeshi]

We can include the test (as is implemented)

[BNAndras]

Before I forget, I noticed you put x:size labels on the issues for adding the PRs. x:size is soft deprecated in favor of x:rep for clarity's sake, but they'll still work for adjusting the reputation awarded to a particular PR. Those labels do need to also be added to the PR directly though. https://exercism.org/docs/using/product/reputation#h-creating-a-pull-request has some examples of when you might use the different labels, but it's purely optional. An unlabeled PR is worth the same rep as a x:rep/medium / x:size/medium PR.

[0xNeshi]

Thx for the tips!