[FEATURE] Output sequence alleles in VCF

[eldariont]

At some point, we should output sequence alleles instead of symbolic alleles in the VCF.

Then, a deletion would no longer look like this:
chr1 57 . N <DEL> 60 PASS END=70;SVLEN=-13;SVTYPE=DEL

Instead, it would contain the complete sequence:
chr1 57 . ACAGATAGATACAG A 60 PASS END=70;SVLEN=-13;SVTYPE=DEL

For this feature, we need the genome file as input. For insertions, we would need to construct a consensus sequence or simply choose one sequence randomly.