Support retrieving an untagged list of repositories in Pagure

So far, the only way I found to retrieve several repositories from a Pagure instance is by having different sections, for example:

```
[rpms/golang]
service = pagure
pagure.repo = rpms/golang
pagure.base_url = https://src.fedoraproject.org
pagure.description_template = {{pagureid}}: {{paguretitle}}
pagure.project_template = Fedora

[rpms/delve]
service = pagure
pagure.repo = rpms/delve
pagure.base_url = https://src.fedoraproject.org
pagure.description_template = {{pagureid}}: {{paguretitle}}
pagure.project_template = Fedora

[rpms/golang-x-oauth2]
service = pagure
pagure.repo = rpms/golang-x-oauth2
pagure.base_url = https://src.fedoraproject.org
pagure.description_template = {{pagureid}}: {{paguretitle}}
pagure.project_template = Fedora
```

The `pagure.include_repos` behaves differently compared to `gitlab.include_repos`. In Pagure, it is only available if you use the `pagure.tag` option, but in GitLab it's the list of repositories to include.

`pagure.repo` only accepts one item, and having to include several sections in the `targets` field is a little hard, as we cannot use regexp nor wildcards (as far as I know).

Oddly, if you use the tag function, and the repository doesn't have the issues activated (like it happens on Fedora's instance), bugwarrior fails:

```
ERROR:bugwarrior.collect:Worker for [Fedora] failed: Failed to talk to 'https://src.fedoraproject.org/api/0/golang/issues' {'error': 'Project not found', 'error_code': 'ENOPROJECT'}
Traceback (most recent call last):
File "/home/alex/Code/src/github.com/GothenburgBitFactory/bugwarrior/bugwarrior/collect.py", line 36, in _aggregate_issues
for issue in service.issues():
File "/home/alex/Code/src/github.com/GothenburgBitFactory/bugwarrior/bugwarrior/services/pagure.py", line 185, in issues
issues.extend(self.get_issues(repo, ('issues', 'issues')))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/alex/Code/src/github.com/GothenburgBitFactory/bugwarrior/bugwarrior/services/pagure.py", line 132, in get_issues
raise OSError('Failed to talk to %r %r' % (url, error))
OSError: Failed to talk to 'https://src.fedoraproject.org/api/0/golang/issues' {'error': 'Project not found', 'error_code': 'ENOPROJECT'}
```

Is there a way to create something like this?:

```
[Fedora]
service = pagure
pagure.include_repos = rpms/golang, rpms/delve, rpms/golang-x-oauth2
pagure.base_url = https://src.fedoraproject.org
pagure.description_template = {{pagureid}}: {{paguretitle}}
pagure.project_template = Fedora

```