Make specifying elements optional in bst commands

Known issues:
* `bst shell` works, but `bst shell COMMANDS...` doesn't, because click
  has no way of separating optional args from variable-length args.
* `bst checkout`'s usage string marks LOCATION as an optional argument.
  Because click gets confused if there's an optional argument before a
  mandatory argument, I had to mark LOCATION as optional internally.
* `bst workspace open` makes no sense with element being optional, so
  I skipped it.
* `bst workspace close` will probably need to be revisited when multiple
  projects can own one workspace.
* `bst workspace reset` will happily delete the directory you're
  currently in, requiring you to `cd $PWD` to see the contents of your
  directory.
  I could exclude the top-level directory of the workspace being
  deleted, but it is entirely valid to run workspace commands from deeper
  in the workspace.
* `bst source-bundle` does not work if a workspace is open at all, and
  according to #672 is scoped for deprecation, so I have left it alone.

This is a part of #222