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
```
Why do you use yield if you're afterwards still going to put them in a list (and as such always retrieve all)?
Yield only makes sense in my opinion if your code can at least continue with the list it got previously, and not if it still has to wait for the full list anyway.