_frontend/cli.py: Add option to close multiple workspaces

At present, it is only possible to close workspaces for elements one at a time. This can become slightly tedious process when you have multiple workspaces open and you want to close all of them, maybe because you just finished working on a set of related elements.

Instead of accepting a single element, accept a list of elements as argument for bst workspace close. Additionally, add -a/--all option to close all workspaces.

Fixes #337 (closed) - Add option to close all workspaces.