Support having a different td subdirectory for different folders on disk

[sweavo]

Currently, you can use td -f . to create a new td database in the current folder. But this means that those issues are not visible when you start td from elsewhere.

When you just use td then you use the default database, but all instances use the same directory stack and current dir.

I'm seeing something like td -c . (c for context) which uses the global task list and directory stack, but a local file stores the current directory. That way, you can have the whole database available, but hitting td in a different sandbox will remind you what you were doing in that sandbox.