Threads handling

Two things:

1. Is there a way to get a list of all currently running threads? This should probably be a `threads` command.

2. It's a bit confusing to understand what `continue` does in a threaded environment. From the `continue` documentation:

> The continue (or c) command continues threads until the next breakpoint occurs.

From the `thread` documentation:

> The thread command switches the debugger to thread ``, making this thread the current thread. The current thread is the thread which is targeted by the following commands: continue, step, next, finish, where, up, down, frame, print, set

The question is: Does `continue` continue execution of all threads or just the current thread? If it's the latter, how do we get back to an "execute all threads" state?