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?

Start by tracing the command handling for `threads`, `thread`, and `continue`, along with the existing command documentation. Determine whether execution is scoped to the current thread or all threads, add the requested thread listing if appropriate, and update the documentation so the behavior and way to resume all threads are explicit.

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