webdiff process should detach

[danvk]

If closing the tab reliably terminates the server, then there's no danger in detaching the webdiff process when you launch it. And it will give you control of your terminal back.

[danvk]

After #223, webdiff does detect the tab close reliably.

"Detaching" seems a little trickier than I expected, though:

• It's not as simple as adding a & to the end of a command like you would with a shell script. Using subprocess.Popen ala this question is the most effective approach I've found so far.
• When the main process terminates, git difftool cleans up the temporary left and right directories that it created. So I'll have to make copies of those and clean them up myself.