Add code to link remote and local paths.

[rsekman]

As discussed in #211. See stig -h link.

At the moment only takes effect for the move command, but that may be the most frequently used. Only absolute paths are allowed in links, so relative paths should pass transparently through move, but I haven't tested this.

It would be nice to have local path/remote path fields in the various torrent views; this shouldn't be too difficult.

[rndusr]

That's much simpler than I thought it would be.

Looks like it should be working great.

[rndusr]

IIRC, run() can be either sync or async. This run() doesn't seem to use the async event loop, so it probably should be sync.

But does it matter?