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 by Tristan Van Berkom
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