[Concepts]: add ternary operator

[meatball133]

@angelikatyborska
@glennj

resolves #387

[glennj]

Does this need to be a separate concept, or can it be mentioned in a Conditionals concept?

[meatball133]

I mean, it could be, I did this mostly to avoid the concept of the conditional for becoming too long and I don't know if the exercise suits using ternary.

I am also planning to add if as a suffix to the about.md on conditionals (will do that this weekend). But I mean if you feel like adding this won't make the concept of the conditional too large, I think we can perhaps merge them

[glennj]

IMO it only needs to be very short:

## Ternary Operator

Crystal implements the familiar ternary operator: `condition ? expression_if_true : expression_if_false`

```crystal
1 == 0 ? "equal" : "not equal"
# => "not equal"
```

[meatball133]

Yeah, I think we can merge them.

I will have to look over it later this week.