Display call allele counts

[timothymillar]

It would be nice display a call_allele_count array in a more intuitive manner, similar to the display_genotypes function (#37). Scikit-allel does this nicely with its GenotypeAlleleCountsArray class which displays ordered counts seperated by :. For example the call_allele_count array

[
    [2, 0, 0], [1, 0, 1], [1, 1, 0]
    [2, 0, 0], [1, 1, 0], [1, 0, 0]
]

would be displayed something like

2:0:0    1:0:1    1:1:0
2:0:0    1:1:0    1:0:0