Ability to show URL associated with task

[jelmer]

It would be great if there was a way to view the URL associated with a task in todoman.

Better yet would be if there was some command to open the URL in a browser.

[WhyNotHugo]

Yeah, this doesn't sound like a big deal; todo show should include the url.

We don't actually show url anywhere, so we'd also have to include this in edit/etc. Summed up, we'd need to:

• [ ] Add url to the model.
• [ ] Add url to the cache.
• [ ] Add url to new/edit.
• [ ] Add url to show.
• [ ] Add url to porcelain.