Merging outputs from Q2-DADA2

[ebolyen (Evan Bolyen)]

@thermokarst

EDIT: There is an open issue on the bug tracker to support variadic inputs, which would theoretically allow a method like feature-table merge to merge multiple tables at the same time.

In the new QIIME 2 2017.12 release, feature-table merge can now accept arbitrarily many tables to merge!