Support Absolute path for merge_plink

[olivierlabayle]

Hi,

Because merge_plink uses glob it does not support absolute paths (here. Would it be possible to add support for it? I think the following works:

dir, prefix = splitdir(prefix)
l = glob(prefix * "*.bed", dir)

Thanks!

[kose-y]

@olivierlabayle Yes, we can definitely consider it.