git credential store for isomorphic-git 1.0.0 based on #377 plugin/event system

As noted in #582 (closed), isomorphic-git no longer has a "core" system. Therefore a different approach to git credential stores is needed. This issue proposes using the event/plugin system from #377 (closed) for this purpose. Whatever the eventual implementation of #377 (closed), this current proposal needs the event emitter to be supplied to pipeline stages or at least the aggregate-content stage.