support for int8

[rborder]

I'd like to read in plink data to int8 arrays rather than float32. Looking at _bed_read.py it seems this could be done at the chunk level by modifying _read_bed_chunk(). Are their any gotchas I'm missing? Happy to contribute this feature

[shz9]

This would be a great feature. I think the rationale for the float32 implementation is that it supports handling for missing values with np.nan. Maybe one workaround would be to set missing values to -1 and let the user handle it from there based on downstream applications.