Support referencing multiple issues from a PR

[iliapolo]

Sometimes, during the implementation of a specific issue, you find yourself changing parts of the code that you didn't initially thought were related. These might even be breaking changes.

Currently, there is no way to reflect those transitive changes in the changelog. It doesn't make sense to edit the original issue, because the changes aren't directly related to it, and should be specified separately.

Imagine you have to add/remove a command line option - this type of change warrants an issue of its own.

One might argue:

In this case, you should split the code and create a separate PR - referencing its own issue

While that is true in theory, in practice, that isn't always so easy. Either because the code is too embedded into other, un-related changes, or just because the changes must be committed together.

A nice solution can be the following:

• Create another issue, specifying the transitive change.
• Add a reference to the new issue in the original PR.

Essentially it means that a single PR can resolve multiple issues - which seems reasonable.

This way, PyCI will detect ALL issues referenced from a PR, and include all of them in the changelog.

We should remember that PyCI also detects issues from the commit message itself, not just the PR body. Should we support specifying multiple issues in the commit message?

[iliapolo]

This issue is part of release 0.8.0