Let .pulls accept just a username.

Before this, it needed both a username and a repo.  Check this out:

```
@threebean │ .pulls
  threebot │ threebean: (pulls <username[/repo]>) -- List the pending pull requests on github repos.
@threebean │ .pulls what/is/this
  threebot │ threebean: Must be GitHub repo of the format <username/repo> or just <username>
@threebean │ .pulls lmacken
  threebot │ threebean: One moment, please...  Looking up lmacken.
  threebot │ threebean: @msabramo's "README.rst: syntax highlight + tweaks" https://github.com/lmacken/tbgrep/pull/8 filed
           │ 4 days ago
  threebot │ threebean: @Qalthos's "Update action_hooks" https://github.com/lmacken/pyramid-openshift-quickstart/pull/7
           │ filed a year ago                                                                                            e
@threebean │ .pulls fedora-infra
  threebot │ threebean: One moment, please...
  threebot │ threebean: @ralphbean's "Handle timezones." https://github.com/fedora-infra/bugzilla2fedmsg/pull/4 filed 41
           │ minutes ago
  threebot │ threebean: @ralphbean's "Remove unused imports." https://github.com/fedora-infra/fedmsg/pull/300 filed 2 days
           │ ago
  threebot │ threebean: @pypingou's "Let the unorphan action call the unorphan API endpoint"
           │ https://github.com/fedora-infra/packagedb-cli/pull/21 filed 3 days ago
  threebot │ threebean: @ralphbean's "Getting ready for the 1.2.0 release." https://github.com/fedora-infra/kitchen/pull/5
           │ filed 4 days ago
  threebot │ threebean: ... and 6 more.
@threebean │ .pulls fedora-infra/fedmsg
  threebot │ threebean: @ralphbean's "Remove unused imports." https://github.com/fedora-infra/fedmsg/pull/300 filed 2 days
           │ ago
```
Ah, how about raising some kind of exception (NotFoundError?), and have the main function catch that and return the "Could not find X" error?