--- Part Two ---

Your calculation isn't quite right. It looks like some of the digits are
actually spelled out with letters: one, two, three, four, five, six, seven,
eight, and nine also count as valid "digits".

Equipped with this new information, you now need to find the real first and last
digit on each line. For example:

```
two1nine
eightwothree
abcone2threexyz
xtwone3four
4nineeightseven2
zoneight234
7pqrstsixteen
```

In this example, the calibration values are 29, 83, 13, 24, 42, 14, and 76.
Adding these together produces 281.

What is the sum of all of the calibration values?