_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 - Add option to close all workspaces.