Allow specifying multiple target elements in every command

Now that we've ditched the variant conditions in favor of project-wide options, this has become possible.

It will be practical to change every command that currently has semantics as:

bst <options> commmand <options> ELEMENT

Instead become:

bst <options> commmand <options> ELEMENT [ELEMENT2 [ELENENT3...]]

Edited Oct 29, 2017 by Tristan Van Berkom