Update changelog tool for https://github.com/nodejs/node/pull/6503

[jasnell]

The scripts/release-post.sh tool will need to be updated to understand the new changelog structure once nodejs/node#6503 lands. The new PR moves the specific changelogs for each release line into separate files in doc/changelogs/*. The pattern is simple: v4 releases will go into doc/changelogs/CHANGELOG_V4.md, v5 releases go into doc/changelogs/CHANGELOG_V5, etc. The specific changelog entries will no longer be listed in the root CHANGELOG.md file.

[evanlucas]

I've been meaning to do this. I ended up just doing v6.2.0 manually. I'll try to get to it in the next few days though.

[evanlucas]

Ah, scratch that, didn't realize @lpinca had already started on it. Thanks!!!!