New Pull Requests: use PR template when available

[guyzmo]

for example, current template could be:

{% title, *body = repository.commit().message.split('\n') %}
### Title
{{ title }}

### Body
{{ '\n'.join(body) }}

{% match = re.match(repository.head.name, r'^(?P<branch_commit>\d+).*') %}
{% if match %}
closes {{ match.groupdict()['branch_commit'] }}
{% endif %}