Allow workspace commands to work on multiple elements

When working on an issue that spans multiple elements, frequently one wants to perform the same operation on multiple workspaces. Naturally, this has 3 parts since there are 3 workspace commands that need to support this:

• bst workspace close: See #337 (closed) and !357 (merged).
• bst workspace reset: See !417 (merged).
• bst workspace open: This is a bit trickier than the last two since the command currently expects the location of the workspace as second argument. I can think of at least these two ways to approach this problem:
  • By allowing users to configure a default location, as described in #229 (closed).
  • Opening the workspaces in sub-directories inside the target directory. Optionally, user may be asked to specify the target location as a format string.

Update by @toscalix : a proposal that covers this tickets has been sent to the mailing list.