Write a function that takes an integer in input and outputs a string with currency format.
Integer in currency format is expressed by a string of number where every three characters are separated by comma. 
For example:
```
123456 --> "123,456"
```
Input will always be an positive integer, so don't worry about type checking or negative/float values.
to_currency = '{:,}'.format
to_currency = '{:,}'.format