triangle: add topics

[ilya-khadykin]

As stated in #808 we need to add topics for exercises. They should be relevant to the exercise even though you might be able to solve it without encountering that topic. Here is the list of commonly used topics in every track - https://github.com/exercism/problem-specifications/blob/master/TOPICS.txt, please use topics from this list or add one if you think it's missing but required.
Hint: you can take a look at an example implementation here (SPOILERS) or in a different language.

There are some exercises which have topics in config.json, so you can look at them as an example.

If you still have questions, I encourage you to ask them before starting working on the issue.

I also suggest claiming the issue by creating an empty PR as discribded here

Please note that there are some special 'closing issue' keywords that you can put in description of your PR and Github will take care of closing the corresponding issue automatically after merge.
This simple thing helps us a lot, and makes our life a bit easier, so please reference the issue in your PR accordingly.