pybigtools: more flexibility with open

[jackh726]

Currently, for files, we hard code the extensions (.bigWig/.bw/.bigBed/.bb). Seems that this is worked around (https://github.com/Bio2Byte/tools-iuc/blob/df9c285dddde7d901823c608c8d7dab971224b5b/tools/bigwig_outlier_bed/bigwig_outlier_bed.py#L115)

Would be good to offer a bit more flexibility. A couple options:

1. Add a type parameter (bigwig, bigbed)
2. Test open a file regardless of extension

[ghuls]

Preferably it would just rely on the first 4 magic bytes to determine if it is a bigwig or bigbed file and not look at the extension at all:
https://github.com/jackh726/bigtools/blame/master/bigtools/src/bbi/bbiread.rs#L657-L664

Probably having that code in a separate function that would return if a file is a bigWig or bigBed file, would be useful.

[jackh726]

Yes, that's covered under (2). Pretty trivial to do.

[jackh726]

I did add IIRC a GenericBBIFile somewhere that does this.