Implement url association

``` js

> todo --g work "read about Relay" --url "https://facebook.github.io/relay/"
// Added task "read about Relay" to group "work" with the URL "https://facebook.github.io/relay/"
> todo open "relay"
// opening "https://facebook.github.io/relay/" in browser...

```

It should use pattern matching to find the appropriate link to open

Start by locating the CLI handling for the `--url` option and the `todo open` command. Use the example in the issue to trace how tasks are stored and matched; done means `todo open` uses pattern matching to find the appropriate task URL and opens it in a browser.

Written by the indexing model from the issue text.