Support for similarity matrices and common variation unit counts in distance/similarity matrices

This release introduces the --table similarity option, which produces a tabular output with counts of pairwise agreements between witnesses (or, if the --proportion flag is specified, proportions of agreements among variation units where both witnesses have non-ambiguous readings), as well as the --show-ext flag, which adds the number of variation units where both witnesses have non-ambiguous readings to each cell's value (e.g., 47/50 or 0.94/50). This option can also be used with distance matrices specified with --table distance.