Drop-down menu for input signatures

[zhiiiyang]

From Reviewer 1. https://f1000research.com/articles/9-586

The authors might consider extending the user-supplied signature mode to allow the user to input a set of signatures, and then select the signature to compare using a drop-down menu (as in the other comparison modes), as it is likely that users will be interested in analyzing sets of signatures, rather than single signatures. However, I understand this might not be straightforward to implement.