Add multiline option

**SAFE TO MERGE** [Open for suggestions]

This new feature allows to display the graph on several lines, as requested in #55. The default is 1 line to preserve compatibility, and the height can be set using the `SPARK_HEIGHT` variable as follows.
```sh
SPARK_HEIGHT=3 ./spark 16 10 7 4 6 7 10
█      
██    █
███▄███
```

**Caveats:**
* It only works with a proper fixed-size font
* There is always a small line skip

Feel free to improve, I am not an expert.