Makes navigating to the PR way easier when looking at the logs.
This requires a new `Forge.PullRequestURL()` method that needs to be
implemented for all forges.
Right now we always show all new releasable commits since the last
_stable_ release.
If a project publishes multiple pre-releases in a row, they all repeat
the same lines with a few new additions.
On the other hand, when we cut a stable release, we do want to show all
changes since the last stable release, as we can not expect users to
read the changelog of pre-releases.
As a compromise, the code now looks at the type of release that is being
created, and decides based on that if we will look at STABLE..HEAD
(stable release) or LATEST..HEAD (pre-release).
Close#33
