Compute Pascal's triangle up to a given number of rows.

In Pascal's Triangle each number is computed by adding the numbers to
the right and left of the current position in the previous row.

```text
    1
   1 1
  1 2 1
 1 3 3 1
1 4 6 4 1
# ... etc
```
## Source

[Pascal's Triangle at Wolfram Math World](http://mathworld.wolfram.com/PascalsTriangle.html)

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.