Crash when loading bigWig file

Hi @kcleal , 

GW (1.2.6) is crashing when trying to load a bigWig file (downloaded from the GTEX db) as a track using `--track Aligned.sortedByCoord.out.patched.md.bigWig`



```
libc++abi: terminating due to uncaught exception of type std::invalid_argument: stoi: no conversion
Abort trap: 6
```

Do you know what could be the problem? 

Thanks! 
Pierre